{
  "format": "stapler-pack",
  "version": 1,
  "kind": "library",
  "meta": {
    "title": "Geometry",
    "author": "",
    "description": "A first-principles Geometry course library: 28 topic categories in hierarchical order, each sitting after the concepts it depends on. Explanations name the misconception behind each wrong answer rather than only marking it wrong.",
    "created": "2026-08-06T04:14:55.997Z",
    "license": ""
  },
  "payload": {
    "library": {
      "name": "Geometry",
      "categories": [
        {
          "id": "logic",
          "title": "Logic",
          "blurb": "Hypotheses, conditionals, converses, and truth tables — the rules of valid inference every geometric proof in this course relies on.",
          "order": 0
        },
        {
          "id": "points-lines-planes",
          "title": "Points, Lines, Line Segments & Planes",
          "blurb": "The undefined terms — point, line, plane — plus segment length, midpoints, and the distance formula used to measure them.",
          "order": 1
        },
        {
          "id": "angles",
          "title": "Angles",
          "blurb": "Angle vocabulary, angle pairs, and the angle-relationship theorems used to justify every proof that follows.",
          "order": 2
        },
        {
          "id": "parallel-perpendicular-lines",
          "title": "Parallel and Perpendicular Lines",
          "blurb": "Transversals, corresponding and alternate angles, and the slope conditions that make two lines parallel or perpendicular.",
          "order": 3
        },
        {
          "id": "coordinate-lines",
          "title": "Lines in the Coordinate Plane",
          "blurb": "Writing and reading linear equations to describe geometric boundaries and relationships on the coordinate plane.",
          "order": 4
        },
        {
          "id": "triangles-review",
          "title": "Triangles Review",
          "blurb": "Classification by side and angle, and the Triangle Inequality — the baseline facts every deeper triangle theorem builds on.",
          "order": 5
        },
        {
          "id": "polygons",
          "title": "Polygons",
          "blurb": "Interior and exterior angle sum theorems for convex polygons of any number of sides.",
          "order": 6
        },
        {
          "id": "congruence-transformations",
          "title": "Congruence Transformations",
          "blurb": "Translations, reflections, and rotations — the rigid motions that preserve a figure's size and shape exactly.",
          "order": 7
        },
        {
          "id": "dilations",
          "title": "Dilations",
          "blurb": "Scale factors and the one non-rigid transformation that preserves shape while changing size.",
          "order": 8
        },
        {
          "id": "symmetry",
          "title": "Symmetry",
          "blurb": "Identifying the lines and points a figure maps onto itself through.",
          "order": 9
        },
        {
          "id": "congruent-figures-review",
          "title": "Congruent Figures Review",
          "blurb": "Recognizing when two figures are congruent from their transformation history, before proving it formally.",
          "order": 10
        },
        {
          "id": "congruent-triangles",
          "title": "Congruent Triangles",
          "blurb": "SSS, SAS, ASA, AAS, and HL — the minimum conditions that guarantee two triangles are identical, and the proofs that use them.",
          "order": 11
        },
        {
          "id": "similar-figures",
          "title": "Similar Figures",
          "blurb": "Proportional sides and equal angles — what similarity guarantees, and the scale factors that connect similar figures.",
          "order": 12
        },
        {
          "id": "properties-of-triangles",
          "title": "Properties of Triangles",
          "blurb": "Medians, altitudes, midsegments, special right triangle ratios, and the points of concurrency triangles produce.",
          "order": 13
        },
        {
          "id": "quadrilaterals",
          "title": "Quadrilaterals",
          "blurb": "Parallelograms, rectangles, rhombi, squares, trapezoids, and kites — the side, angle, and diagonal relationships that define each.",
          "order": 14
        },
        {
          "id": "pythagorean-theorem",
          "title": "Pythagorean Theorem",
          "blurb": "The right-triangle relationship a² + b² = c², and the Pythagorean triples it produces.",
          "order": 15
        },
        {
          "id": "right-triangle-trig",
          "title": "Right Triangle Trigonometry",
          "blurb": "SOH-CAH-TOA and inverse trig functions for finding a missing side or angle in a right triangle.",
          "order": 16
        },
        {
          "id": "laws-of-sines-cosines",
          "title": "Laws of Sines and Cosines",
          "blurb": "Extending trigonometry to triangles that don't have a right angle.",
          "order": 17
        },
        {
          "id": "area-perimeter",
          "title": "Area and Perimeter",
          "blurb": "Area and perimeter formulas for two-dimensional figures, from rectangles to compound shapes.",
          "order": 18
        },
        {
          "id": "three-d-figures",
          "title": "Three-Dimensional Figures",
          "blurb": "Identifying and describing prisms, cylinders, pyramids, cones, and spheres before measuring them.",
          "order": 19
        },
        {
          "id": "surface-area-volume",
          "title": "Surface Area and Volume",
          "blurb": "Lateral area, total surface area, and volume for the solids introduced in Three-Dimensional Figures.",
          "order": 20
        },
        {
          "id": "measurement",
          "title": "Measurement",
          "blurb": "Unit conversion and precision — the practical measurement skills geometry problems assume.",
          "order": 21
        },
        {
          "id": "circles",
          "title": "Circles",
          "blurb": "Central and inscribed angles, chords, secants, tangents, arc length, and sector area.",
          "order": 22
        },
        {
          "id": "constructions-circles",
          "title": "Constructions with Circles",
          "blurb": "Compass-and-straightedge constructions involving circles — inscribed and circumscribed figures, tangent lines.",
          "order": 23
        },
        {
          "id": "circles-coordinate-plane",
          "title": "Circles in the Coordinate Plane",
          "blurb": "Deriving and using the standard equation of a circle from its center and radius.",
          "order": 24
        },
        {
          "id": "parabolas-coordinate-plane",
          "title": "Parabolas in the Coordinate Plane",
          "blurb": "The geometric definition of a parabola — a focus and a directrix — and its standard equation on the coordinate plane.",
          "order": 25
        },
        {
          "id": "probability",
          "title": "Probability",
          "blurb": "Geometric probability — using length, area, and volume to measure likelihood.",
          "order": 26
        },
        {
          "id": "vectors",
          "title": "Vectors",
          "blurb": "Magnitude and direction, vector addition, and using vectors to describe motion geometrically.",
          "order": 27
        }
      ],
      "quizzes": [
        {
          "id": "logic-foundations",
          "title": "Logic Foundations",
          "blurb": "Conditionals, negation and De Morgan's Laws, quantifiers, the contrapositive, and the postulate structure every later proof leans on.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "logic"
          ],
          "questions": [
            {
              "id": "log-001",
              "topic": "Conditional Statements",
              "difficulty": "medium",
              "stem": "Evaluate the truth value of the following conditional statement: “If a triangle has four sides, then squares are circles.”",
              "choices": [
                "True",
                "False",
                "Undefined",
                "Neither — it is a biconditional"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A conditional is false in exactly one situation: a true hypothesis with a false conclusion. Here the hypothesis (“a triangle has four sides”) is false, so that situation never arises and the statement is vacuously true. Nothing about squares or circles has to be checked.",
                "False is the intuitive answer, and it comes from grading the conclusion on its own — “squares are circles” is obviously wrong, so the whole thing feels wrong. But a conditional does not assert its conclusion; it only promises the conclusion whenever the hypothesis holds. That promise is never tested here.",
                "Undefined is not one of the values a conditional can take. Every combination of hypothesis and conclusion truth values has a defined result in the truth table — that is what makes truth tables finite and checkable.",
                "A biconditional is “p if and only if q”, which asserts the implication in both directions. This statement says “if… then…” once, in one direction, so it is a plain conditional."
              ],
              "hint": "Look at the hypothesis before the conclusion. What does a conditional promise when its hypothesis is false?"
            },
            {
              "id": "log-002",
              "topic": "Negation and De Morgan",
              "difficulty": "medium",
              "stem": "By De Morgan's Laws, what is the logical equivalent of the negation ¬(p ∧ q)?",
              "choices": [
                "¬p ∨ ¬q",
                "p ∨ q",
                "¬p ∧ ¬q",
                "q → p"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Negating an “and” turns it into an “or” of the negated parts: ¬(p ∧ q) ≡ ¬p ∨ ¬q. For the pair to fail, it is enough that one of them fails — you are not required to say which.",
                "p ∨ q drops the negation entirely. This is the disjunction of the original statements, not of their denials, so it is true in cases where ¬(p ∧ q) is false.",
                "¬p ∧ ¬q negates both parts but keeps the “and”, which is the classic De Morgan slip. It claims both fail. ¬(p ∧ q) only claims that they do not both hold — a much weaker statement, satisfied when exactly one fails.",
                "q → p is a conditional, and De Morgan's Laws are about distributing a negation across ∧ and ∨. No implication appears in ¬(p ∧ q)."
              ],
              "hint": "The negation flips the connective as well as the parts. Ask what has to go wrong for “both p and q” to be untrue — one failure, or two?"
            },
            {
              "id": "log-003",
              "topic": "Quantifiers",
              "difficulty": "medium",
              "stem": "What is the correct logical negation of the universal statement “All angles are acute”?",
              "choices": [
                "At least one angle is not acute.",
                "All angles are obtuse.",
                "Some angles are obtuse.",
                "No angles are acute."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The negation of “all A are B” is “at least one A is not B”. To disprove a universal claim you only have to produce one counterexample — a single non-acute angle is enough, and it need not be obtuse (a right angle would do).",
                "“All angles are obtuse” is another universal claim, not the denial of the first one. Both statements can be false together, which a statement and its negation can never be.",
                "“Some angles are obtuse” is too specific in one direction and too weak in another. A counterexample only has to be non-acute; right angles are non-acute without being obtuse, so this misses cases where the original is genuinely false.",
                "“No angles are acute” is the strongest possible opposite — it denies every instance. The negation only needs to deny one."
              ],
              "hint": "You are trying to break a claim about everything. What is the smallest amount of evidence that does it?"
            },
            {
              "id": "log-004",
              "topic": "Converse, Inverse, Contrapositive",
              "difficulty": "medium",
              "stem": "Consider the true statement “If two angles are vertical, then they are congruent.” Which related statement must also be true?",
              "choices": [
                "If two angles are congruent, then they are vertical.",
                "If two angles are not congruent, then they are not vertical.",
                "If two angles are not vertical, then they are not congruent.",
                "Two angles are vertical if and only if they are congruent."
              ],
              "correctIndex": 1,
              "explanations": [
                "This is the converse, and it is false: two angles can measure 40° each without sitting opposite one another at an intersection. A true conditional says nothing about its converse.",
                "Correct. This is the contrapositive, ¬q → ¬p, which is logically equivalent to the original conditional — the two are true and false together, always. It is why proof by contrapositive is a valid technique rather than a trick.",
                "This is the inverse, ¬p → ¬q. Like the converse it is not equivalent to the original: plenty of non-vertical angle pairs are congruent, so it is false here.",
                "A biconditional requires the conditional AND its converse to hold. The converse fails here, so the biconditional fails with it."
              ],
              "hint": "Of the converse, inverse, and contrapositive, exactly one is always equivalent to the original. Which one negates both parts and swaps them?"
            },
            {
              "id": "log-005",
              "topic": "Conditional Statements",
              "difficulty": "easy",
              "stem": "In the conditional “If a figure is a square, then it is a rectangle”, which part is the hypothesis?",
              "choices": [
                "“it is a rectangle”",
                "“a figure is a square”",
                "The whole statement",
                "Neither — a conditional has no hypothesis"
              ],
              "correctIndex": 1,
              "explanations": [
                "“It is a rectangle” follows “then”, which makes it the conclusion. The conclusion is what the statement promises, not what triggers the promise.",
                "Correct. The hypothesis is the clause introduced by “if” — the condition that has to hold before the statement claims anything. Here that is “a figure is a square”.",
                "A conditional is built from two distinct clauses. Calling the whole statement the hypothesis loses the structure that makes it testable.",
                "Every conditional has both a hypothesis and a conclusion by definition; that is what makes it a conditional rather than a simple assertion."
              ],
              "hint": "One clause follows “if” and the other follows “then”. Which one sets the condition?"
            },
            {
              "id": "log-006",
              "topic": "Valid Inference",
              "difficulty": "medium",
              "stem": "You know “If it is raining, then the field is wet” is true, and you observe that the field is wet. What can you validly conclude?",
              "choices": [
                "It is raining.",
                "It is not raining.",
                "Nothing about the rain.",
                "It rained earlier but has stopped."
              ],
              "correctIndex": 2,
              "explanations": [
                "Concluding the hypothesis from the conclusion is affirming the consequent, and it is invalid. The conditional rules out rain-with-a-dry-field; it never claims rain is the only way a field gets wet.",
                "Nothing licenses denying the rain either. The observation is consistent with rain and with no rain, so it cannot establish either one.",
                "Correct. A true conditional plus a true conclusion tells you nothing about the hypothesis. Sprinklers, a burst pipe, or rain all leave the field wet, and the statement does not distinguish between them. (What WOULD be valid: a dry field would let you conclude it is not raining — that is the contrapositive.)",
                "This adds a detail the premises never supplied. Nothing here mentions timing at all."
              ],
              "hint": "Work out which direction of the conditional you are travelling in, and check whether that direction is the one the statement actually guarantees."
            },
            {
              "id": "log-007",
              "topic": "Postulates and Proof",
              "difficulty": "medium",
              "stem": "Which of Euclid's five postulates uniquely defines the geometry of a flat plane, guaranteeing that parallel lines never meet?",
              "choices": [
                "The Fifth Postulate (the Parallel Postulate)",
                "The Fourth Postulate",
                "The Third Postulate",
                "The First Postulate"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The Fifth Postulate is independent of the first four: they hold on a sphere and on a saddle too, where “parallel” behaves quite differently. Adding the fifth is exactly what pins the geometry down to a flat plane — which is why dropping it produces the non-Euclidean geometries rather than a contradiction.",
                "The Fourth Postulate states that all right angles are equal to one another. It fixes a common standard for angle measure, and it holds in curved geometries as well.",
                "The Third Postulate says a circle can be drawn with any centre and radius. It is a construction rule and carries no information about parallels.",
                "The First Postulate says a straight line can be drawn between any two points. It is the most basic of the five and is satisfied in curved geometries too."
              ],
              "hint": "Four of the five survive unchanged on a curved surface. The odd one out is the one that fails there."
            },
            {
              "id": "log-008",
              "topic": "Negation and De Morgan",
              "difficulty": "hard",
              "stem": "A theorem states “If a quadrilateral is a rhombus, then its diagonals are perpendicular.” To disprove it by counterexample, what must a single figure satisfy?",
              "choices": [
                "It is a rhombus and its diagonals are perpendicular.",
                "It is a rhombus and its diagonals are not perpendicular.",
                "It is not a rhombus and its diagonals are not perpendicular.",
                "It is not a rhombus and its diagonals are perpendicular."
              ],
              "correctIndex": 1,
              "explanations": [
                "This figure satisfies the theorem rather than breaking it. An example that obeys a claim is not evidence against it, however many you collect.",
                "Correct. ¬(p → q) is equivalent to p ∧ ¬q — the one and only case a conditional forbids. A counterexample must therefore meet the hypothesis and fail the conclusion simultaneously. (No such rhombus exists, which is why the theorem stands.)",
                "Failing the hypothesis makes the conditional vacuously true for that figure, so this cannot be a counterexample. It is outside the theorem's scope entirely.",
                "A non-rhombus with perpendicular diagonals — a kite, say — is perfectly compatible with the theorem. The theorem never claimed rhombi were the only such figures; that would be its converse."
              ],
              "hint": "Write the negation of p → q as a conjunction first. A conditional is false in exactly one row of its truth table — recreate that row."
            },
            {
              "id": "log-009",
              "topic": "Valid Inference",
              "difficulty": "hard",
              "stem": "Given “If a figure is a square, then it is a rectangle” and “If a figure is a rectangle, then it has four right angles”, what does the Law of Syllogism let you conclude?",
              "choices": [
                "If a figure has four right angles, then it is a square.",
                "If a figure is a square, then it has four right angles.",
                "If a figure is a rectangle, then it is a square.",
                "Every figure with four right angles is a rectangle."
              ],
              "correctIndex": 1,
              "explanations": [
                "This reverses the chain. Syllogism carries you forward from the first hypothesis to the last conclusion, never backward — and the reversal is false anyway, since a non-square rectangle has four right angles.",
                "Correct. The Law of Syllogism chains p → q with q → r to give p → r. Here p is “is a square”, q is “is a rectangle”, and r is “has four right angles”, so the chain yields “square → four right angles”.",
                "This is the converse of the first premise, which syllogism does not license. A rectangle need not be a square.",
                "This is the converse of the second premise. True as it happens, but it does not follow from these premises by syllogism — it would need to be established separately."
              ],
              "hint": "Line the two conditionals up so the conclusion of one is the hypothesis of the other, then read off the two ends."
            },
            {
              "id": "log-010",
              "topic": "Conditional Statements",
              "difficulty": "easy",
              "stem": "A conditional statement p → q is FALSE in exactly which case?",
              "choices": [
                "p is true and q is true",
                "p is false and q is true",
                "p is true and q is false",
                "p is false and q is false"
              ],
              "correctIndex": 2,
              "explanations": [
                "Both true is the case the statement was designed to describe — the promise was made and kept, so the conditional holds.",
                "A false hypothesis makes the conditional vacuously true regardless of q. The promise was never triggered, so it cannot have been broken.",
                "Correct. A conditional is a promise: “whenever p, then q”. The only way to break it is for p to happen and q to fail. Every other row of the truth table is true.",
                "Both false is again a vacuous case — the hypothesis never fired, so nothing was promised and nothing was broken."
              ],
              "hint": "Read p → q as a promise and ask what it would take to accuse someone of breaking it."
            },
            {
              "id": "log-011",
              "topic": "Postulates and Proof",
              "difficulty": "medium",
              "stem": "In a two-stage congruence proof, what is the primary role of CPCTC (Corresponding Parts of Congruent Triangles are Congruent)?",
              "choices": [
                "To prove that two lines are parallel.",
                "To verify that the Triangle Inequality holds.",
                "To bisect an existing angle.",
                "To extract a specific pair of equal sides or angles once congruence is already established."
              ],
              "correctIndex": 3,
              "explanations": [
                "Parallelism is proved from angle relationships across a transversal. CPCTC may supply one of those angle equalities, but that is CPCTC being used, not what it is.",
                "The Triangle Inequality is a fact about side lengths in any single triangle and needs no congruence at all.",
                "Bisecting is a construction. CPCTC proves things about a figure that already exists; it does not build anything.",
                "Correct. CPCTC is not itself a criterion — it is what you harvest after a criterion has done its job. The logic of nearly every congruence proof runs in two beats: first establish △ ≅ △ by SSS, SAS, ASA, AAS or HL, then quote CPCTC to collect the one equal side or angle you actually wanted. It gives you all six correspondences for free."
              ],
              "hint": "Ask whether it is used to establish congruence or to spend it once you have it."
            },
            {
              "id": "log-012",
              "topic": "Converse, Inverse, Contrapositive",
              "difficulty": "medium",
              "stem": "The statement “If two angles form a linear pair, then they are supplementary” is true, and so is its converse-style partner “If two adjacent angles are supplementary and share a common ray, they form a linear pair.” What does this pair of facts justify writing?",
              "choices": [
                "A biconditional: angles form a linear pair if and only if they are adjacent, share a ray, and are supplementary.",
                "Only the original conditional; the converse adds nothing.",
                "The inverse of the original statement.",
                "Nothing — a statement and its converse cannot both be true."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. When a conditional and its converse are both true, the two conditions are interchangeable, and that is precisely what a biconditional (“if and only if”) records. Definitions in geometry are biconditional for this reason.",
                "The converse adds a great deal: it lets you argue in the other direction, from supplementary-and-adjacent back to linear pair. That is a second inference the original alone does not license.",
                "The inverse negates both parts of the original. It is equivalent to the converse, not to the pair of them, and it is not what having both directions buys you.",
                "A statement and its converse can absolutely both be true — that is the ordinary situation for a definition. They are simply independent of one another, so each has to be checked separately."
              ],
              "hint": "You now have the implication running in both directions. Is there a single connective that says exactly that?"
            },
            {
              "id": "log-013",
              "topic": "Contrapositive",
              "difficulty": "easy",
              "stem": "Given the statement \"If it rains, then the game is cancelled,\" which of the following is its contrapositive?",
              "choices": [
                "If the game is cancelled, then it rained.",
                "If it does not rain, then the game is not cancelled.",
                "If the game is not cancelled, then it did not rain.",
                "It rains and the game is not cancelled."
              ],
              "correctIndex": 2,
              "explanations": [
                "This is the CONVERSE — it swaps the hypothesis and conclusion without negating either. The converse is not logically equivalent to the original: the game could be cancelled for a reason other than rain, which would make this false while the original stays true.",
                "This is the INVERSE — it negates both parts but leaves them in their original order. Like the converse, it is not equivalent to the original, and for the same reason: some other cause could still cancel the game.",
                "Correct. The contrapositive negates BOTH parts AND swaps them: ¬q → ¬p. It is the only one of the three related conditionals that is logically equivalent to the original, which is why proving the contrapositive is a legitimate way to prove the original.",
                "This is the NEGATION of the conditional, not a related conditional at all. \"If p then q\" is false exactly when p is true and q is false, so this statement asserts the original is false rather than restating it."
              ],
              "hint": "Of converse, inverse and contrapositive, only one is logically equivalent to the original. Ask which operations — swapping, negating, or both — produce it."
            },
            {
              "id": "log-014",
              "topic": "Logical Equivalence",
              "difficulty": "medium",
              "stem": "A conditional statement is known to be true. Which related statement must also be true?",
              "choices": [
                "Its converse",
                "Its inverse",
                "Its contrapositive",
                "All three must be true"
              ],
              "correctIndex": 2,
              "explanations": [
                "The converse (q → p) can be false while the original is true. \"If a figure is a square, then it is a rectangle\" is true; its converse \"if a rectangle, then a square\" is false. One counterexample settles it.",
                "The inverse (¬p → ¬q) can likewise be false. \"If not a square, then not a rectangle\" fails for any non-square rectangle. The inverse is the contrapositive of the converse, so it stands or falls with the converse — not with the original.",
                "Correct. Only the contrapositive (¬q → ¬p) is logically equivalent to the original. Every row of the truth table matches, so the two statements are true in exactly the same circumstances.",
                "The converse and inverse are equivalent to each other but not to the original, so a true conditional guarantees nothing about them. Assuming it does is the error behind most invalid geometric arguments."
              ],
              "hint": "Test each with a statement you know to be true but whose reverse is false — squares and rectangles work well."
            },
            {
              "id": "log-015",
              "topic": "Properties of Equality",
              "difficulty": "easy",
              "stem": "In a two-column proof, what reason justifies the step \"AB ≅ AB\"?",
              "choices": [
                "Symmetric Property",
                "Reflexive Property",
                "Transitive Property",
                "Segment Addition Postulate"
              ],
              "correctIndex": 1,
              "explanations": [
                "The Symmetric Property says that if a ≅ b then b ≅ a — it reverses a congruence you already have. Here nothing is being reversed; a single segment is being compared with itself.",
                "Correct. The Reflexive Property states that any quantity is congruent to itself. It looks trivial, but it is the reason a shared side counts as a pair of congruent sides when proving two triangles congruent by SSS or SAS.",
                "The Transitive Property chains two congruences: if a ≅ b and b ≅ c then a ≅ c. It needs two given statements to link, and here there is only one segment.",
                "The Segment Addition Postulate says that if B is between A and C then AB + BC = AC. It is about combining lengths along a segment, not about a segment's relationship to itself."
              ],
              "hint": "The step compares one object with itself. Which property is the one that permits that?"
            },
            {
              "id": "log-016",
              "topic": "CPCTC",
              "difficulty": "medium",
              "stem": "After proving △ABC ≅ △DEF, a proof concludes that ∠A ≅ ∠D. What reason justifies that step?",
              "choices": [
                "CPCTC",
                "ASA",
                "The Reflexive Property",
                "The definition of congruent angles"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. CPCTC — Corresponding Parts of Congruent Triangles are Congruent — is the reason that lets you harvest individual congruent sides and angles AFTER the triangles themselves have been proved congruent. The order matters: CPCTC is a consequence of congruence, never a way to establish it.",
                "ASA is a congruence criterion — one of the tools used to PROVE the triangles congruent in the first place. Using it here would be circular, since the triangle congruence is already established and is what the conclusion rests on.",
                "The Reflexive Property applies when an object is compared with itself. ∠A and ∠D belong to two different triangles, so nothing is being compared with itself.",
                "The definition of congruent angles says congruent angles have equal measure. That unpacks what ∠A ≅ ∠D means; it does not supply the reason the relationship holds here."
              ],
              "hint": "The triangles are already congruent. What licenses moving from whole-triangle congruence down to one pair of parts?"
            },
            {
              "id": "log-017",
              "topic": "Law of Syllogism",
              "difficulty": "hard",
              "stem": "Given \"If a figure is a square, then it is a rhombus\" and \"If a figure is a rhombus, then its diagonals are perpendicular,\" what does the Law of Syllogism let you conclude?",
              "choices": [
                "The figure is a square.",
                "If a figure is a square, then its diagonals are perpendicular.",
                "If a figure's diagonals are perpendicular, then it is a square.",
                "Nothing — the two statements are unrelated."
              ],
              "correctIndex": 1,
              "explanations": [
                "This would be the Law of Detachment, and it needs something the problem does not supply: a statement that some specific figure actually IS a square. Syllogism chains conditionals together; detachment fires a single conditional using a confirmed hypothesis.",
                "Correct. The Law of Syllogism chains p → q with q → r to give p → r. The conclusion of the first statement is exactly the hypothesis of the second, so the two link into a single conditional running from square to perpendicular diagonals.",
                "This reverses the chain, producing the converse of the correct conclusion. Perpendicular diagonals do not force a square — a kite has them too — so the reversed statement is actually false.",
                "They are precisely related: the conclusion of the first ('is a rhombus') matches the hypothesis of the second. That shared middle term is the hinge the Law of Syllogism turns on."
              ],
              "hint": "Look for a statement that appears as the conclusion of one conditional and the hypothesis of the other. That shared term is what gets eliminated."
            },
            {
              "id": "log-018",
              "topic": "Reasoning: Conditional Logic",
              "difficulty": "hard",
              "stem": "\"If a figure is a square, then it is a rectangle\" is true. A student concludes that \"if a figure is a rectangle, then it is a square\" must also be true. What is the error?",
              "choices": [
                "They formed the CONVERSE, which is not guaranteed by the original statement.",
                "They formed the contrapositive, which is never true.",
                "They negated only the hypothesis, forming the inverse, which is always true.",
                "There is no error; a conditional and its reverse always agree."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Swapping hypothesis and conclusion gives the converse, and a true conditional says nothing about its converse. A long rectangle is a counterexample.",
                "The contrapositive swaps AND negates, and it is always equivalent to the original — it is the one form that is guaranteed true.",
                "The inverse negates both parts without swapping, and it is not guaranteed true either. This student swapped rather than negated.",
                "A conditional and its converse are independent. Only the contrapositive is logically equivalent to the original."
              ],
              "hint": "Which of the four related conditionals is the only one guaranteed to share the original's truth value?",
              "kind": "reasoning"
            },
            {
              "id": "log-019",
              "topic": "Reasoning: What's Missing",
              "difficulty": "medium",
              "stem": "A proof states: \"∠1 ≅ ∠2 because they are vertical angles.\" What must be true of the figure for this justification to hold?",
              "choices": [
                "The two angles must be supplementary.",
                "Two lines must actually intersect, with the angles opposite each other at that intersection.",
                "The angles must both be acute.",
                "The two angles must share a common side."
              ],
              "correctIndex": 1,
              "explanations": [
                "Supplementary angles sum to 180°. Vertical angles are congruent, which is a different relationship — and supplementary angles are usually not congruent.",
                "Correct. Vertical angles are defined by a crossing: two lines meeting at a point create two pairs of opposite angles. Without an actual intersection there are no vertical angles to appeal to.",
                "Vertical angles can be acute, right or obtuse. Their size is not part of the definition.",
                "Angles that share a side are ADJACENT. Vertical angles share only the vertex and lie opposite one another."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "points-lines-planes",
          "title": "Points, Lines & Planes",
          "blurb": "The undefined terms, betweenness and segment addition, distance and detour bounds, the Triangle Inequality, and what a pair of points actually determines.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "points-lines-planes"
          ],
          "questions": [
            {
              "id": "plp-001",
              "topic": "Distance and Detours",
              "difficulty": "medium",
              "stem": "Let P = (0, 0), Q = (4, 3), and M = (8, 0). A route P → Q → M has a surcharge δ equal to the extra distance travelled compared with going straight from P to M. Find δ(P → Q → M).",
              "choices": [
                "2",
                "10",
                "5",
                "0"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The surcharge is d(P,Q) + d(Q,M) − d(P,M). By the distance formula d(P,Q) = √(4² + 3²) = 5 and d(Q,M) = √(4² + 3²) = 5, while the direct route d(P,M) = 8. So δ = 5 + 5 − 8 = 2.",
                "10 is the total distance actually travelled, 5 + 5. The surcharge is the extra you paid over the direct route, so the direct distance still has to be subtracted.",
                "5 is the length of one leg, not the detour cost. Both legs contribute, and the straight-line distance comes off the total.",
                "0 would mean the detour cost nothing — which happens exactly when Q lies on segment PM. Here Q = (4,3) sits well above the x-axis that P and M lie on, so the route genuinely bends."
              ],
              "hint": "Add the two legs, then subtract what the direct trip would have cost. Both legs are 3-4-5 right triangles.",
              "diagram": "<svg viewBox=\"0 0 240 130\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"30\" y1=\"100\" x2=\"210\" y2=\"100\" stroke=\"#6b6b76\" stroke-width=\"1.6\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"100\" x2=\"120\" y2=\"30\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"30\" x2=\"210\" y2=\"100\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><circle cx=\"30\" cy=\"100\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"120\" cy=\"30\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"210\" cy=\"100\" r=\"3.2\" fill=\"#7c3aed\"/><text x=\"18\" y=\"116\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">P (0,0)</text><text x=\"100\" y=\"22\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">Q (4,3)</text><text x=\"186\" y=\"116\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">M (8,0)</text><text x=\"56\" y=\"60\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5</text><text x=\"174\" y=\"60\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5</text><text x=\"112\" y=\"116\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">8</text></svg>",
              "diagramCaption": "Solid: the route through Q. Dashed: the direct route. Not drawn to scale"
            },
            {
              "id": "plp-002",
              "topic": "Betweenness",
              "difficulty": "medium",
              "stem": "What does it mean geometrically if a waypoint Q is a “free waypoint”, so that δ(P → Q → M) = 0?",
              "choices": [
                "Q forms a right triangle with P and M.",
                "Q lies anywhere on the infinite line containing P and M.",
                "Q lies exactly on the line segment PM.",
                "Q is equidistant from P and M."
              ],
              "correctIndex": 2,
              "explanations": [
                "A right triangle still has three distinct vertices, so the two legs together are strictly longer than the hypotenuse and the surcharge is positive. A right angle at Q makes the detour efficient, not free.",
                "The infinite line is too generous. If Q sits on the line but beyond M, you overshoot and then double back — d(P,Q) + d(Q,M) exceeds d(P,M), so δ > 0. Only the portion between P and M is free.",
                "Correct. By the Straightness Law, the surcharge is zero if and only if the waypoint lies on the segment connecting the endpoints. This is the Segment Addition Postulate stated as a cost: PQ + QM = PM exactly when Q is between P and M.",
                "Equidistant points form the perpendicular bisector of PM, which is an entire line of points. Only one of them — the midpoint — is actually on the segment and therefore free; the rest cost extra."
              ],
              "hint": "Zero extra distance means the two legs add up to exactly the direct distance. Which points Q make PQ + QM = PM, and which make it more?",
              "diagram": "<svg viewBox=\"0 0 240 118\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"60\" x2=\"215\" y2=\"60\" stroke=\"#7c3aed\" stroke-width=\"2.4\" stroke-linecap=\"round\"/><circle cx=\"25\" cy=\"60\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"215\" cy=\"60\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"120\" cy=\"60\" r=\"3.2\" fill=\"#1f2024\"/><text x=\"16\" y=\"48\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">P</text><text x=\"210\" y=\"48\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">M</text><text x=\"112\" y=\"48\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">Q</text><text x=\"52\" y=\"82\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">PQ</text><text x=\"154\" y=\"82\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">QM</text><text x=\"86\" y=\"104\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">PQ + QM = PM</text></svg>",
              "diagramCaption": "A free waypoint sits on the segment itself. Not drawn to scale"
            },
            {
              "id": "plp-003",
              "topic": "Triangle Inequality",
              "difficulty": "easy",
              "stem": "Given three lengths — 5 cm, 7 cm, and 15 cm — why can these not form a valid triangle?",
              "choices": [
                "Because 15 is an odd number.",
                "Because they do not satisfy the Pythagorean theorem.",
                "Because they violate the Triangle Inequality: 5 + 7 < 15.",
                "Because 15 − 7 > 5."
              ],
              "correctIndex": 2,
              "explanations": [
                "Parity has nothing to do with it. Plenty of valid triangles have odd side lengths — 5, 7, 9 works perfectly well.",
                "The Pythagorean theorem is a test for right triangles specifically, not a requirement for triangles in general. Most triangles fail it and exist anyway.",
                "Correct. A triangle requires the sum of any two sides to be strictly greater than the third. Here 5 + 7 = 12, which is less than 15, so the two short sides cannot reach across the long one — they fall short by 3 cm no matter how you angle them.",
                "This inequality is true (8 > 5), and it is a real consequence of the same failure, but it is the difference form of the test rather than the reason. The direct statement is that the two shorter sides must together exceed the longest."
              ],
              "hint": "Try laying the two shorter sticks along the longest one. Do they reach?",
              "diagram": "<svg viewBox=\"0 0 240 106\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"80\" x2=\"205\" y2=\"80\" stroke=\"#6b6b76\" stroke-width=\"2.4\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"55\" x2=\"85\" y2=\"55\" stroke=\"#7c3aed\" stroke-width=\"2.4\" stroke-linecap=\"round\"/><line x1=\"85\" y1=\"55\" x2=\"169\" y2=\"55\" stroke=\"#7c3aed\" stroke-width=\"2.4\" stroke-linecap=\"round\"/><line x1=\"169\" y1=\"55\" x2=\"205\" y2=\"55\" stroke=\"#c0392b\" stroke-width=\"1.6\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"94\" y=\"96\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">15 cm</text><text x=\"40\" y=\"44\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5 cm</text><text x=\"112\" y=\"44\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">7 cm</text><text x=\"160\" y=\"30\" fill=\"#c0392b\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">gap</text></svg>",
              "diagramCaption": "The two shorter sides laid end to end still fall short of the longest. Not drawn to scale"
            },
            {
              "id": "plp-004",
              "topic": "Distance and Detours",
              "difficulty": "hard",
              "stem": "The Surcharge Bound states that a detour can never cost more than twice the shorter of its two legs. If d(P, Q) = 4 and d(Q, M) = 9, what is the maximum possible surcharge δ(P → Q → M)?",
              "choices": [
                "13",
                "18",
                "8",
                "5"
              ],
              "correctIndex": 2,
              "explanations": [
                "13 is d(P,Q) + d(Q,M), the total distance travelled. The surcharge subtracts the direct distance from that total, and the direct distance is never zero here.",
                "18 is twice the LONGER leg. The bound is set by the shorter one — the worst case is doubling back along the short leg, which cannot cost more than that leg twice over.",
                "Correct. The bound is 2·min(d(P,Q), d(Q,M)) = 2 × 4 = 8. The worst case is the fully degenerate detour where Q lies on the far side of P: you walk 4 out, then 9 back past your start, and the excess over the direct 5 is exactly 8.",
                "5 is the difference 9 − 4, which is the SMALLEST the direct distance d(P,M) can be, not the largest the surcharge can be. It is the other end of the same triangle inequality."
              ],
              "hint": "The bound depends on only one of the two legs. Which one, and what multiple of it?"
            },
            {
              "id": "plp-005",
              "topic": "Points and Lines",
              "difficulty": "medium",
              "stem": "How many distinct straight lines can be drawn through 6 points in general position (no three of them collinear)?",
              "choices": [
                "15",
                "20",
                "30",
                "12"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Two points determine exactly one line, so the count is the number of ways to choose 2 points from 6: C(6,2) = 6·5/2 = 15. Counting by hand gives the same thing — the first point pairs with 5 others, the second with 4 new ones, and so on: 5 + 4 + 3 + 2 + 1 = 15.",
                "20 is C(6,3), the number of ways to choose three points — which counts triangles, not lines. A line is fixed by two points, and asking for a third over-specifies it.",
                "30 is 6 × 5, the number of ORDERED pairs. Every line gets counted twice that way, once from each endpoint, so the result has to be halved.",
                "12 is 6 × 2. There is no reading of the problem this corresponds to; the pairing structure is what governs the count."
              ],
              "hint": "How many points does it take to pin down a line? Then count how many such sets 6 points contain — remembering that order does not matter.",
              "diagram": "<svg viewBox=\"0 0 240 140\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"40\" y1=\"30\" x2=\"200\" y2=\"45\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"30\" x2=\"60\" y2=\"110\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"30\" x2=\"150\" y2=\"105\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"200\" y1=\"45\" x2=\"60\" y2=\"110\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"200\" y1=\"45\" x2=\"150\" y2=\"105\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"60\" y1=\"110\" x2=\"150\" y2=\"105\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"30\" x2=\"120\" y2=\"15\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"200\" y1=\"45\" x2=\"120\" y2=\"15\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"60\" y1=\"110\" x2=\"120\" y2=\"15\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"150\" y1=\"105\" x2=\"120\" y2=\"15\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"30\" x2=\"205\" y2=\"95\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"200\" y1=\"45\" x2=\"205\" y2=\"95\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"60\" y1=\"110\" x2=\"205\" y2=\"95\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"150\" y1=\"105\" x2=\"205\" y2=\"95\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"15\" x2=\"205\" y2=\"95\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><circle cx=\"40\" cy=\"30\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"200\" cy=\"45\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"60\" cy=\"110\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"150\" cy=\"105\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"120\" cy=\"15\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"205\" cy=\"95\" r=\"3.2\" fill=\"#7c3aed\"/><text x=\"96\" y=\"130\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">6 points, every pair joined</text></svg>",
              "diagramCaption": "Every pair of points determines one line. Not drawn to scale"
            },
            {
              "id": "plp-006",
              "topic": "Points and Lines",
              "difficulty": "hard",
              "stem": "In a convex octagon (8 sides), how many interior crossing points do the diagonals create, assuming no three diagonals meet at the same point?",
              "choices": [
                "28",
                "70",
                "56",
                "120"
              ],
              "correctIndex": 1,
              "explanations": [
                "28 is C(8,2), the number of total possible connections between vertices — every side and diagonal counted once. That is a different quantity: it counts the segments themselves, not the points where they cross inside.",
                "Correct. Every interior crossing corresponds uniquely to a set of 4 vertices: choose any four, and the two diagonals of the quadrilateral they form cross exactly once. So the count is C(8,4) = (8·7·6·5)/(4·3·2·1) = 70.",
                "56 is 8·7, the number of ordered vertex pairs. Crossings are governed by sets of four vertices, not pairs.",
                "120 is C(10,3) and does not correspond to any structure in an octagon. Check which size of subset a single crossing determines."
              ],
              "hint": "Pick one crossing and ask how many vertices it takes to describe it. Then count subsets of that size."
            },
            {
              "id": "plp-007",
              "topic": "Points and Lines",
              "difficulty": "medium",
              "stem": "Using the diagonal formula d(n) = n(n − 3)/2, how many diagonals does a convex decagon (10 sides) have?",
              "choices": [
                "20",
                "35",
                "45",
                "50"
              ],
              "correctIndex": 1,
              "explanations": [
                "20 would come from n(n − 3)/2 with n = 8, or from halving 40. Substituting n = 10 gives 10 × 7, not 10 × 4.",
                "Correct. Substituting n = 10 gives d(10) = 10(10 − 3)/2 = 10 × 7 / 2 = 70/2 = 35. The formula itself is C(n,2) − n: all the segments joining vertices, minus the n that are sides.",
                "45 is C(10,2) = 45, every segment joining two vertices. That total still includes the 10 sides, which are not diagonals — subtracting them leaves 35.",
                "50 is 10 × 5, which would follow from using n − 5 in place of n − 3. Each vertex joins to n − 3 others by a diagonal: all the vertices except itself and its two neighbours."
              ],
              "hint": "Substitute n = 10 carefully, and divide by 2 only once — each diagonal is counted from both of its ends."
            },
            {
              "id": "plp-008",
              "topic": "Angle Basics",
              "difficulty": "easy",
              "stem": "Two adjacent angles form a straight line. If one angle measures 115°, what is the measure of the other?",
              "choices": [
                "65°",
                "75°",
                "25°",
                "115°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Two adjacent angles on a straight line form a linear pair, and a linear pair sums to 180°. So the other angle is 180° − 115° = 65°.",
                "75° would follow from subtracting 115 from 190. A straight line measures 180°, not 190°.",
                "25° is 90° − 65°, the complement of the correct answer. Complementary means summing to 90°, which describes a right angle rather than a straight line.",
                "115° repeats the given angle. Angles in a linear pair are equal only in the special case where both are 90°, and 115° is not 90°."
              ],
              "hint": "A straight line is a 180° angle, and the two adjacent angles divide it between them.",
              "diagram": "<svg viewBox=\"0 0 240 105\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"20\" y1=\"85\" x2=\"220\" y2=\"85\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"85\" x2=\"62\" y2=\"25\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"66\" y=\"76\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">115°</text><text x=\"140\" y=\"76\" fill=\"#6b6b76\" font-size=\"14\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><circle cx=\"120\" cy=\"85\" r=\"3.2\" fill=\"#7c3aed\"/></svg>",
              "diagramCaption": "The two angles form a linear pair. Not drawn to scale"
            },
            {
              "id": "plp-009",
              "topic": "Angle Basics",
              "difficulty": "easy",
              "stem": "What relationship always holds true for vertical angles?",
              "choices": [
                "They are congruent (equal in measure).",
                "They are supplementary.",
                "They are complementary.",
                "They share a common side."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Vertical angles sit opposite each other at an intersection of two lines, and they must be equal in measure. Each one is the supplement of the same adjacent angle, so the two are forced to match.",
                "Supplementary describes the ADJACENT pairs at that intersection — each vertical angle and the angle beside it. The opposite pair is equal, and equal pairs sum to 180° only in the perpendicular case.",
                "Complementary means summing to 90°. Nothing at a general intersection of two lines guarantees that.",
                "Vertical angles are opposite one another and share only the vertex point, not a side. The pairs that share a side are the adjacent, supplementary ones."
              ],
              "hint": "Both members of the pair are supplementary to the same neighbouring angle. What does that force?",
              "diagram": "<svg viewBox=\"0 0 240 122\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"25\" x2=\"215\" y2=\"105\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"105\" x2=\"215\" y2=\"25\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"108\" y=\"32\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">a</text><text x=\"108\" y=\"108\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">a</text><text x=\"40\" y=\"70\" fill=\"#6b6b76\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">b</text><text x=\"196\" y=\"70\" fill=\"#6b6b76\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">b</text><circle cx=\"120\" cy=\"65\" r=\"3.2\" fill=\"#7c3aed\"/></svg>",
              "diagramCaption": "Opposite angles at an intersection are congruent. Not drawn to scale"
            },
            {
              "id": "plp-010",
              "topic": "Distance and Detours",
              "difficulty": "medium",
              "stem": "Point B lies between A and C on a line. If AB = 3x − 4, BC = x + 6, and AC = 22, what is the value of x?",
              "choices": [
                "5",
                "6",
                "8",
                "20"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Because B is between A and C, the Segment Addition Postulate gives AB + BC = AC. So (3x − 4) + (x + 6) = 22, which simplifies to 4x + 2 = 22, giving 4x = 20 and x = 5.",
                "6 comes from dropping the +2: solving 4x = 24 rather than 4x + 2 = 22. The constants −4 and +6 combine to +2, which has to move across before dividing.",
                "8 would follow from setting the two expressions equal to each other (3x − 4 = x + 6 gives x = 5, not 8) — and in any case B being between A and C does not make the two pieces equal. That would require B to be the midpoint.",
                "20 is 4x, a value that appears mid-solution but is not x itself. One more division is needed."
              ],
              "hint": "Betweenness is what licenses adding the two pieces. Write that equation before touching the algebra.",
              "diagram": "<svg viewBox=\"0 0 240 118\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"60\" x2=\"215\" y2=\"60\" stroke=\"#7c3aed\" stroke-width=\"2.4\" stroke-linecap=\"round\"/><circle cx=\"25\" cy=\"60\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"130\" cy=\"60\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"215\" cy=\"60\" r=\"3.2\" fill=\"#7c3aed\"/><text x=\"18\" y=\"48\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">A</text><text x=\"124\" y=\"48\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">B</text><text x=\"210\" y=\"48\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">C</text><text x=\"48\" y=\"82\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3x − 4</text><text x=\"146\" y=\"82\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x + 6</text><text x=\"104\" y=\"104\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">AC = 22</text></svg>",
              "diagramCaption": "B is between A and C. Not drawn to scale"
            },
            {
              "id": "plp-011",
              "topic": "Undefined Terms",
              "difficulty": "easy",
              "stem": "Point, line, and plane are called the “undefined terms” of geometry. Why are they left undefined?",
              "choices": [
                "Because mathematicians have not yet agreed on definitions for them.",
                "Because any definition would have to use other terms, which would need defining in turn — so the chain has to start somewhere.",
                "Because they cannot be drawn accurately.",
                "Because they only exist in the coordinate plane."
              ],
              "correctIndex": 1,
              "explanations": [
                "There is no outstanding disagreement here. Leaving them undefined is a deliberate structural choice made at the foundation of the subject, not an unfinished argument.",
                "Correct. Every definition is written in terms of something else. If nothing were taken as primitive, the definitions would either run forever or loop back on themselves. Point, line and plane are the agreed starting points: they are described by how they behave in the postulates rather than defined from anything prior.",
                "Drawing accuracy is a practical matter and has nothing to do with definability. A circle cannot be drawn perfectly either, and it is defined precisely.",
                "These terms predate coordinates by roughly two thousand years and apply just as well in synthetic geometry, where no coordinate system exists at all."
              ],
              "hint": "Try actually defining “point” without using any geometric word that would itself need defining."
            },
            {
              "id": "plp-012",
              "topic": "Undefined Terms",
              "difficulty": "medium",
              "stem": "Three points are chosen at random in space. Which statement is guaranteed to be true?",
              "choices": [
                "They always determine exactly one plane.",
                "They always lie on exactly one line.",
                "There is at least one plane containing all three.",
                "They always form a triangle."
              ],
              "correctIndex": 2,
              "explanations": [
                "“Exactly one” fails when the three points happen to be collinear: infinitely many planes contain a single line, so no one plane is picked out. The postulate requires the three points to be non-collinear before it promises uniqueness.",
                "Three points lie on one line only if they happen to be collinear, which random points generally are not. Two points always determine a line; three do not.",
                "Correct. Some plane always contains all three — if they are non-collinear it is unique, and if they are collinear there are infinitely many to choose from. Existence holds in both cases; only uniqueness needs the non-collinear condition.",
                "Collinear points form a degenerate figure with no area, not a triangle. A triangle needs three non-collinear vertices."
              ],
              "hint": "Separate two different claims — that such a plane exists, and that only one does. The collinear case breaks exactly one of them."
            },
            {
              "id": "plp-013",
              "topic": "Determining a Plane",
              "difficulty": "easy",
              "stem": "How many non-collinear points are required to determine exactly one plane?",
              "choices": [
                "Two",
                "Three",
                "Four",
                "Any number, as long as they are distinct"
              ],
              "correctIndex": 1,
              "explanations": [
                "Two points determine exactly one LINE, not a plane. Infinitely many planes contain any given line — picture a book's pages all sharing the spine.",
                "Correct. Three non-collinear points determine exactly one plane. The non-collinear condition is doing real work: three points on a single line still leave infinitely many planes containing them, which is why a three-legged stool never wobbles but its legs must not be in a row.",
                "Four points generally determine NO single plane — a fourth point usually sits off the plane fixed by the first three. Four points lying in one plane is a special condition called coplanarity, not a general fact.",
                "Distinctness is not enough. Three distinct but collinear points fail to pin down a plane, and more than three distinct points usually cannot lie in one plane at all."
              ],
              "hint": "Ask what two points determine first, then what has to be added — and what condition those points must satisfy."
            },
            {
              "id": "plp-014",
              "topic": "Intersecting Planes",
              "difficulty": "medium",
              "stem": "Two distinct planes intersect. What is their intersection?",
              "choices": [
                "A single point",
                "A line",
                "A plane",
                "They cannot intersect"
              ],
              "correctIndex": 1,
              "explanations": [
                "A single point would require the planes to touch and immediately separate, which flat unbounded surfaces cannot do. Once two planes share one point they necessarily share a whole line through it.",
                "Correct. If two distinct planes intersect at all, their intersection is exactly one line — the postulate behind the crease where two walls meet. Sharing a point forces sharing the entire line through it.",
                "Sharing a whole plane would make the two planes identical, contradicting the premise that they are distinct. Coincident planes are one plane, not two.",
                "Distinct planes certainly can intersect. The only planes that fail to intersect are parallel ones, which is a special case, not the general rule."
              ],
              "hint": "Think about where two walls of a room meet, and what shape that meeting is."
            },
            {
              "id": "plp-015",
              "topic": "Segment Addition",
              "difficulty": "easy",
              "stem": "Point B lies between A and C on a line. If AB = 3x − 1, BC = x + 5, and AC = 28, find x.",
              "choices": [
                "4",
                "6",
                "7",
                "8"
              ],
              "correctIndex": 1,
              "explanations": [
                "x = 4 gives AB = 11 and BC = 9, totalling 20 rather than 28. This comes from solving 4x + 4 = 20 — using the wrong total.",
                "Correct. By the Segment Addition Postulate, AB + BC = AC. So (3x − 1) + (x + 5) = 28, giving 4x + 4 = 28, then 4x = 24 and x = 6. Check: AB = 17, BC = 11, and 17 + 11 = 28.",
                "x = 7 gives AB = 20 and BC = 12, summing to 32 — an overshoot. This usually comes from dropping the constant +4 and solving 4x = 28.",
                "x = 8 gives AB = 23 and BC = 13, summing to 36. This is far past the given total and suggests the two expressions were combined incorrectly before solving."
              ],
              "hint": "Betweenness turns into addition: the two shorter pieces must sum to the whole."
            },
            {
              "id": "plp-016",
              "topic": "Midpoint",
              "difficulty": "medium",
              "stem": "A segment has midpoint (3, 4) and one endpoint at (1, 2). Find the other endpoint.",
              "choices": [
                "(2, 3)",
                "(5, 6)",
                "(4, 6)",
                "(−1, 0)"
              ],
              "correctIndex": 1,
              "explanations": [
                "(2, 3) is the midpoint of (1,2) and (3,4) — it treats the given midpoint as though it were the second endpoint. The midpoint is the destination of the averaging, not an input to it.",
                "Correct. The midpoint is the average of the endpoints, so each coordinate of the endpoint satisfies x = 2(3) − 1 = 5 and y = 2(4) − 2 = 6. Check: the midpoint of (1,2) and (5,6) is ((1+5)/2, (2+6)/2) = (3,4).",
                "(4, 6) has the right y but adds the run instead of doubling: from (1,2) the step to (3,4) is (+2,+2), so the far endpoint is (3+2, 4+2) = (5,6), not (4,6).",
                "(−1, 0) steps backwards from the given endpoint rather than forwards through the midpoint. It would put both given points on the same side of the answer."
              ],
              "hint": "The midpoint sits halfway. Whatever step takes you from the endpoint to the midpoint must be repeated once more."
            },
            {
              "id": "plp-017",
              "topic": "Collinearity",
              "difficulty": "hard",
              "stem": "Points A(1, 2), B(4, 8), and C(6, 12) are given. Which statement is true?",
              "choices": [
                "All three are collinear.",
                "Only A and B lie on a line through the origin.",
                "A, B and C form a triangle with positive area.",
                "B is the midpoint of AC."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Collinearity is tested by comparing slopes: AB = (8−2)/(4−1) = 2, BC = (12−8)/(6−4) = 2, and AC = (12−2)/(6−1) = 2. All three agree, and every point satisfies y = 2x, so the three lie on one line.",
                "A line through the origin has the form y = mx. Both A and B satisfy y = 2x — but so does C, since 2(6) = 12. The word 'only' is what makes this false; all three points are on that line.",
                "A triangle needs three NON-collinear vertices. Since all three slopes come out to 2, these points lie on a single line and enclose zero area — a degenerate triangle, not a real one.",
                "For B to be the midpoint of AC it would have to sit at ((1+6)/2, (2+12)/2) = (3.5, 7). B is at (4, 8), so it lies on the segment but not at its middle."
              ],
              "hint": "Compute the slope between each pair of points. Equal slopes throughout mean the points share a single line."
            },
            {
              "id": "plp-018",
              "topic": "Reasoning: Definitions",
              "difficulty": "medium",
              "stem": "A student says three points always determine exactly one plane. When is that claim false?",
              "choices": [
                "Never — three points always determine exactly one plane.",
                "When the three points are collinear, in which case infinitely many planes contain them.",
                "When the points are very far apart.",
                "When two of the points coincide, in which case no plane exists."
              ],
              "correctIndex": 1,
              "explanations": [
                "The postulate requires the three points to be NON-collinear, which the student's statement leaves out.",
                "Correct. Three collinear points lie on a single line, and infinitely many planes can be rotated about that line. The postulate is that three NON-collinear points determine exactly one plane.",
                "Distance is irrelevant. A plane is unbounded, so points can be any distance apart.",
                "Two coincident points still lie in planes — infinitely many, since it reduces to two distinct points at most."
              ],
              "hint": "State the postulate exactly. Which word is the student leaving out?",
              "kind": "reasoning"
            },
            {
              "id": "plp-019",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "Given that M is between A and B with AM = 7 and MB = 5, a student writes AB = 7 − 5 = 2. What is the error?",
              "choices": [
                "Betweenness means the parts ADD to the whole: AB = 7 + 5 = 12.",
                "The student should have used the midpoint formula.",
                "AB cannot be found without coordinates.",
                "Nothing is wrong; AB = 2."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The Segment Addition Postulate says that if M is between A and B, then AM + MB = AB. So AB = 12.",
                "The midpoint formula finds a location from coordinates. M is not stated to be a midpoint here, and no coordinates are given.",
                "Coordinates are not needed. The two part-lengths and the betweenness relationship are enough.",
                "AB = 2 would be shorter than AM alone, which is impossible when M lies between A and B."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "angles-mastery",
          "title": "Angles Mastery & Calculation",
          "blurb": "Angle pairs, parallel-line transversals, polygon angle sums, and triangle angle relationships — 30 questions from foundational to competitive.",
          "meta": "37 questions · easy → hard",
          "categories": [
            "angles",
            "parallel-perpendicular-lines",
            "triangles-review",
            "polygons"
          ],
          "questions": [
            {
              "id": "ang-001",
              "topic": "Angle Pairs",
              "difficulty": "easy",
              "stem": "Two angles are supplementary, and their measures are in the ratio 3:7. What is the measure of the smaller angle?",
              "choices": [
                "18°",
                "54°",
                "126°",
                "36°"
              ],
              "correctIndex": 1,
              "explanations": [
                "18° is the value of x, not an angle. Writing the angles as 3x and 7x gives 10x = 180, so x = 18 — but the question asks for an angle, which is 3x. Solving for the variable and stopping is the most common way to lose this question.",
                "Correct. Supplementary angles sum to 180°, so 3x + 7x = 180, giving x = 18. The smaller angle is 3(18) = 54°.",
                "126° is 7x — the larger angle. The ratio 3:7 puts the smaller measure on the 3 part, so this answers the opposite of what was asked.",
                "36° comes from treating the angles as complementary (summing to 90°): 10x = 90 would give x = 9 and 3x = 27, and 36° doesn't follow from either reading. Supplementary means a straight line, 180°, not a right angle."
              ],
              "hint": "Give the two angles a shared variable — 3x and 7x — and use what supplementary means about their sum.",
              "diagram": "<svg viewBox=\"0 0 240 110\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"20\" y1=\"90\" x2=\"220\" y2=\"90\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"90\" x2=\"60\" y2=\"25\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><path d=\"M 96 90 A 26 26 0 0 0 106 72\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><path d=\"M 150 90 A 32 32 0 0 0 140 66\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"84\" y=\"80\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3x</text><text x=\"152\" y=\"78\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">7x</text><circle cx=\"120\" cy=\"90\" r=\"3\" fill=\"#7c3aed\"/></svg>",
              "diagramCaption": "The two angles form a straight line. Not drawn to scale"
            },
            {
              "id": "ang-002",
              "topic": "Angle Pairs",
              "difficulty": "medium",
              "stem": "The measure of an angle is 24° more than twice the measure of its complement. Find the measure of the angle.",
              "choices": [
                "22°",
                "44°",
                "68°",
                "112°"
              ],
              "correctIndex": 2,
              "explanations": [
                "22° is the complement of the correct answer (90 − 68). It answers a question that wasn't asked — the angle itself was requested, not its complement.",
                "44° results from dropping the 24°: solving x = 2(90 − x) gives 3x = 180 and x = 60, and 44° doesn't follow from that either. The '24° more than' has to be added to the doubled complement.",
                "Correct. The complement is 90 − x, so x = 2(90 − x) + 24. That expands to x = 204 − 2x, giving 3x = 204 and x = 68°.",
                "112° comes from using the supplement instead of the complement: x = 2(180 − x) + 24 heads toward a value above 90°. Complementary angles sum to 90°, not 180°."
              ],
              "hint": "Complementary angles sum to 90°, so if the angle is x, its complement is 90 − x. Translate 'more than twice' directly into that expression."
            },
            {
              "id": "ang-003",
              "topic": "Angle Pairs",
              "difficulty": "easy",
              "stem": "Angles A and B form a linear pair. If m∠A = 4x + 12 and m∠B = 6x − 22, what is the value of x?",
              "choices": [
                "10",
                "19",
                "20",
                "22"
              ],
              "correctIndex": 1,
              "explanations": [
                "10 comes from setting the two expressions equal to each other (4x + 12 = 6x − 22 gives x = 17, not 10) or from a slip in combining terms. A linear pair is not a congruent pair — the angles add to 180°, they don't match.",
                "Correct. A linear pair sums to 180°: (4x + 12) + (6x − 22) = 180 simplifies to 10x − 10 = 180, so 10x = 190 and x = 19.",
                "20 is what you get by dropping the −10 and solving 10x = 200. The constants +12 and −22 combine to −10, which has to be moved across before dividing.",
                "22 comes from summing to 200 rather than 180, or from adding the constants as +34 instead of −10. Check the sign on the −22 when combining."
              ],
              "hint": "A linear pair sits on a straight line. Add the two expressions and set the total to what a straight line measures.",
              "diagram": "<svg viewBox=\"0 0 240 110\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"20\" y1=\"90\" x2=\"220\" y2=\"90\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"90\" x2=\"165\" y2=\"25\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"60\" y=\"80\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4x + 12</text><text x=\"170\" y=\"78\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">6x − 22</text><circle cx=\"120\" cy=\"90\" r=\"3\" fill=\"#7c3aed\"/></svg>",
              "diagramCaption": "∠A and ∠B form a linear pair. Not drawn to scale"
            },
            {
              "id": "ang-004",
              "topic": "Angle Pairs",
              "difficulty": "medium",
              "stem": "Two lines intersect. One angle measures 5x − 18 and the vertical angle opposite it measures 3x + 14. What is the measure of one of the remaining obtuse angles?",
              "choices": [
                "16°",
                "62°",
                "118°",
                "128°"
              ],
              "correctIndex": 2,
              "explanations": [
                "16° is the value of x, not an angle measure. It has to be substituted back into one of the expressions before it means anything geometric.",
                "62° is the vertical angle pair itself: 3(16) + 14 = 62°. But 62° is acute, and the question asks for one of the remaining obtuse angles — the ones adjacent to it.",
                "Correct. Vertical angles are congruent, so 5x − 18 = 3x + 14 gives x = 16 and an angle of 62°. The adjacent angle forms a linear pair with it: 180° − 62° = 118°.",
                "128° would be the answer if the 62° angle were subtracted from a straight line incorrectly, or if x were taken as 18. 180 − 62 = 118, not 128."
              ],
              "hint": "Vertical angles are equal — that gives you x. Then remember the obtuse angles are the ones next to that pair, not the pair itself.",
              "diagram": "<svg viewBox=\"0 0 240 125\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"25\" x2=\"215\" y2=\"105\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"105\" x2=\"215\" y2=\"25\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"92\" y=\"30\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5x − 18</text><text x=\"92\" y=\"108\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3x + 14</text><text x=\"38\" y=\"70\" fill=\"#6b6b76\" font-size=\"14\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><text x=\"196\" y=\"70\" fill=\"#6b6b76\" font-size=\"14\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><circle cx=\"120\" cy=\"65\" r=\"3\" fill=\"#7c3aed\"/></svg>",
              "diagramCaption": "The two marked angles are vertical angles; the ? angles are the obtuse pair. Not drawn to scale"
            },
            {
              "id": "ang-005",
              "topic": "Angle Pairs",
              "difficulty": "medium",
              "stem": "Ray BD bisects ∠ABC. If m∠ABD = x² − 6 and m∠DBC = 5x + 8, find the positive value of x.",
              "choices": [
                "2",
                "7",
                "8",
                "14"
              ],
              "correctIndex": 1,
              "explanations": [
                "2 is the absolute value of the negative root. Factoring gives (x − 7)(x + 2) = 0, so the roots are 7 and −2. A negative root is discarded here because it would produce a negative angle measure, but its sign can't simply be flipped.",
                "Correct. A bisector splits the angle into two equal parts, so x² − 6 = 5x + 8. Rearranging gives x² − 5x − 14 = 0, which factors to (x − 7)(x + 2) = 0. The positive root is x = 7.",
                "8 comes from a factoring slip — looking for numbers that multiply to −14 and add to −5 gives −7 and +2, not −8 and something. Substituting 8 gives 58 and 48, which aren't equal.",
                "14 is the constant from the quadratic, not a root. It appears in x² − 5x − 14 = 0 as the product of the roots, not as a solution itself."
              ],
              "hint": "A bisector produces two equal angles. Set the expressions equal and you'll get a quadratic — only one root can be an angle measure.",
              "diagram": "<svg viewBox=\"0 0 240 120\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"40\" y1=\"100\" x2=\"200\" y2=\"100\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"100\" x2=\"165\" y2=\"22\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"100\" x2=\"190\" y2=\"62\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"96\" y=\"92\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x² − 6</text><text x=\"104\" y=\"60\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5x + 8</text><text x=\"196\" y=\"20\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">A</text><text x=\"198\" y=\"60\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">D</text><text x=\"206\" y=\"104\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">C</text><text x=\"28\" y=\"104\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">B</text><circle cx=\"40\" cy=\"100\" r=\"3\" fill=\"#7c3aed\"/></svg>",
              "diagramCaption": "Ray BD bisects ∠ABC. Not drawn to scale"
            },
            {
              "id": "ang-006",
              "topic": "Angles Around a Point",
              "difficulty": "medium",
              "stem": "Three rays from a common vertex form three adjacent angles that together make a full revolution. Their measures are consecutive multiples of 5. What is the measure of the largest angle?",
              "choices": [
                "115°",
                "120°",
                "125°",
                "130°"
              ],
              "correctIndex": 2,
              "explanations": [
                "115° is the smallest of the three angles (5n where n = 23), not the largest. The three measures are 115°, 120°, and 125°.",
                "120° is the middle angle. It's also the answer you'd get by assuming the three angles are equal (360 ÷ 3), which ignores the 'consecutive multiples' condition entirely.",
                "Correct. Write the angles as 5n, 5n + 5, and 5n + 10. A full revolution is 360°, so 15n + 15 = 360, giving n = 23. The largest is 5(23) + 10 = 125°.",
                "130° would follow if the angles were spaced by 10 rather than 5, or if the revolution were taken as 375°. Consecutive multiples of 5 differ by exactly 5."
              ],
              "hint": "Angles filling a complete turn about a point sum to 360°. Write the three consecutive multiples with one variable.",
              "diagram": "<svg viewBox=\"0 0 240 140\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><circle cx=\"120\" cy=\"70\" r=\"3\" fill=\"#7c3aed\"/><line x1=\"120\" y1=\"70\" x2=\"220\" y2=\"70\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"70\" x2=\"55\" y2=\"15\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"70\" x2=\"70\" y2=\"125\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"158\" y=\"52\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5n</text><text x=\"74\" y=\"62\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5n + 5</text><text x=\"128\" y=\"116\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5n + 10</text></svg>",
              "diagramCaption": "Three angles about a single point. Not drawn to scale"
            },
            {
              "id": "ang-007",
              "topic": "Parallel Lines & Transversals",
              "difficulty": "medium",
              "stem": "Lines l and m are parallel, cut by transversal t. A pair of same-side interior angles measure 7x + 16 and 4x − 1. What is the measure of the smaller angle?",
              "choices": [
                "15°",
                "59°",
                "121°",
                "60°"
              ],
              "correctIndex": 1,
              "explanations": [
                "15° is the value of x. It still has to be substituted into one of the expressions to become an angle.",
                "Correct. Same-side interior angles between parallel lines are supplementary: (7x + 16) + (4x − 1) = 180 gives 11x + 15 = 180, so x = 15. The angles are 121° and 59°, and the smaller is 59°.",
                "121° is the larger angle, 7(15) + 16. The question asks for the smaller of the pair.",
                "60° comes from treating the two angles as congruent instead of supplementary: 7x + 16 = 4x − 1 gives a negative x. Same-side interior angles are supplementary — it's the alternate interior pair that's congruent."
              ],
              "hint": "Same-side interior angles sit on the same side of the transversal, between the parallels. Decide whether that makes them equal or supplementary before writing an equation.",
              "diagram": "<svg viewBox=\"0 0 240 140\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"15\" y1=\"35\" x2=\"225\" y2=\"35\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"15\" y1=\"105\" x2=\"225\" y2=\"105\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"55\" y1=\"15\" x2=\"175\" y2=\"125\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"78\" y=\"55\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">7x + 16</text><text x=\"96\" y=\"98\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4x − 1</text><text x=\"226\" y=\"39\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">l</text><text x=\"226\" y=\"109\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">m</text><text x=\"178\" y=\"130\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">t</text></svg>",
              "diagramCaption": "l ∥ m, cut by transversal t. Not drawn to scale"
            },
            {
              "id": "ang-008",
              "topic": "Parallel Lines & Transversals",
              "difficulty": "hard",
              "stem": "Two horizontal lines are parallel. A path bends between them, forming a zigzag. The upper segment makes a 35° angle with the top line and the lower segment makes a 45° angle with the bottom line. What is the measure of the bend angle z?",
              "choices": [
                "70°",
                "80°",
                "100°",
                "280°"
              ],
              "correctIndex": 1,
              "explanations": [
                "70° doubles the 35° instead of combining both given angles. Only one of the two deflections is 35°; the other is 45°.",
                "Correct. Draw an auxiliary line through the vertex parallel to both lines. It splits z into two parts, each an alternate interior angle with one of the given angles. So z = 35° + 45° = 80°.",
                "100° treats one of the angles as supplementary before adding — for example 180 − 35 − 45. The auxiliary line makes each part directly equal to a given angle, with no subtraction involved.",
                "280° is the reflex angle on the outside of the bend, 360° − 80°. The angle marked z is the interior one at the bend."
              ],
              "hint": "Draw a line through the bend parallel to the other two. Each half of z then pairs with one of the given angles.",
              "diagram": "<svg viewBox=\"0 0 240 135\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"15\" y1=\"25\" x2=\"225\" y2=\"25\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"15\" y1=\"115\" x2=\"225\" y2=\"115\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><polyline points=\"20,25 150,70 20,115\" fill=\"none\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linejoin=\"round\"/><text x=\"38\" y=\"42\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">35°</text><text x=\"38\" y=\"108\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">45°</text><text x=\"156\" y=\"74\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">z</text><circle cx=\"150\" cy=\"70\" r=\"3\" fill=\"#1f2024\"/></svg>",
              "diagramCaption": "The two horizontal lines are parallel. Not drawn to scale"
            },
            {
              "id": "ang-009",
              "topic": "Parallel Lines & Transversals",
              "difficulty": "medium",
              "stem": "A pair of alternate exterior angles formed by a transversal across parallel lines measure 3x + 40 and 5x − 10. What is the measure of the acute angle adjacent to them?",
              "choices": [
                "25°",
                "65°",
                "115°",
                "50°"
              ],
              "correctIndex": 1,
              "explanations": [
                "25° is the value of x, not an angle measure.",
                "Correct. Alternate exterior angles are congruent: 3x + 40 = 5x − 10 gives x = 25, so each measures 115°. The adjacent angle forms a linear pair with it: 180° − 115° = 65°.",
                "115° is the alternate exterior angle itself. The question asks for the acute angle beside it, and 115° is obtuse.",
                "50° is 2x, which appears while solving (2x = 50) but is not an angle in the figure."
              ],
              "hint": "Alternate exterior angles are congruent — that gives x. Then use a linear pair to step to the adjacent angle."
            },
            {
              "id": "ang-010",
              "topic": "Polygon Angle Sums",
              "difficulty": "medium",
              "stem": "The interior angles of a convex pentagon are x, 2x, 1.5x, 1.8x, and 2.7x. What is the measure of the smallest interior angle?",
              "choices": [
                "60°",
                "67.5°",
                "90°",
                "54°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A pentagon's interior angles sum to (5 − 2) × 180 = 540°. The coefficients total 9, so 9x = 540 and x = 60°. The smallest angle is x itself.",
                "67.5° comes from dividing 540 by 8 instead of 9. Adding the coefficients 1 + 2 + 1.5 + 1.8 + 2.7 gives 9.",
                "90° would follow from using 720° as the pentagon's sum (the hexagon value) — 720 ÷ 8 = 90. A pentagon has n = 5, so (n − 2) × 180 = 540°.",
                "54° divides 540 by 10 rather than 9. It's also close enough to the correct answer to slip past a quick check, so re-add the coefficients."
              ],
              "hint": "Use (n − 2) × 180 for the total, then add the coefficients of x before dividing."
            },
            {
              "id": "ang-011",
              "topic": "Polygon Angle Sums",
              "difficulty": "medium",
              "stem": "Each interior angle of a regular convex polygon measures 168°. How many sides does the polygon have?",
              "choices": [
                "20",
                "24",
                "30",
                "36"
              ],
              "correctIndex": 2,
              "explanations": [
                "20 would correspond to an exterior angle of 18°, giving an interior angle of 162°, not 168°.",
                "24 corresponds to an exterior angle of 15° and an interior angle of 165°. Close, but the exterior angle here is 12°.",
                "Correct. Each exterior angle is 180° − 168° = 12°. Exterior angles of any convex polygon sum to 360°, so n = 360 ÷ 12 = 30.",
                "36 would come from dividing 360 by 10. The exterior angle is 12°, not 10°."
              ],
              "hint": "Go through the exterior angle. It's the supplement of the interior angle, and the exterior angles always total 360°."
            },
            {
              "id": "ang-012",
              "topic": "Polygon Angle Sums",
              "difficulty": "hard",
              "stem": "The ratio of an interior angle to an exterior angle of a regular polygon is 5:1. How many diagonals does the polygon have?",
              "choices": [
                "12",
                "54",
                "65",
                "90"
              ],
              "correctIndex": 1,
              "explanations": [
                "12 is the number of sides, not the number of diagonals. It's the halfway point of the problem — the diagonal formula still has to be applied.",
                "Correct. Interior and exterior angles are supplementary, so 5x + x = 180 gives x = 30° for the exterior angle. Then n = 360 ÷ 30 = 12 sides, and the number of diagonals is n(n − 3)/2 = 12(9)/2 = 54.",
                "65 comes from using n(n − 3)/2 with n = 13, or from n(n − 1)/2 with a slip. Each vertex connects to n − 3 others by a diagonal — not n − 1, since a vertex joins to itself and its two neighbours by sides, not diagonals.",
                "90 is n(n − 3) = 12 × 9 without halving. Every diagonal gets counted twice that way, once from each endpoint."
              ],
              "hint": "The ratio gives you the exterior angle, which gives you n. Then a separate formula turns n into a diagonal count."
            },
            {
              "id": "ang-013",
              "topic": "Triangle Angle Relationships",
              "difficulty": "medium",
              "stem": "In △ABC, the exterior angle at vertex C measures 142°. If m∠A = 3x + 10 and m∠B = 5x − 4, what is m∠A?",
              "choices": [
                "17",
                "61°",
                "81°",
                "38°"
              ],
              "correctIndex": 1,
              "explanations": [
                "17 is the value of x. It carries no degree sign because it isn't an angle — substituting it into 3x + 10 is the step that's missing.",
                "Correct. The Exterior Angle Theorem says the exterior angle equals the sum of the two remote interior angles: (3x + 10) + (5x − 4) = 142 gives 8x + 6 = 142, so x = 17 and m∠A = 3(17) + 10 = 61°.",
                "81° is m∠B, which is 5(17) − 4. The question asks for m∠A.",
                "38° is the interior angle at C, the supplement of 142°. That's a real angle in the figure, but it isn't ∠A."
              ],
              "hint": "The exterior angle equals the sum of the two interior angles not adjacent to it — no need to find the third angle first.",
              "diagram": "<svg viewBox=\"0 0 240 130\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"30\" y1=\"105\" x2=\"175\" y2=\"105\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"175\" y1=\"105\" x2=\"225\" y2=\"105\" stroke=\"#6b6b76\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"105\" x2=\"110\" y2=\"25\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"110\" y1=\"25\" x2=\"175\" y2=\"105\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"40\" y=\"96\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3x + 10</text><text x=\"96\" y=\"44\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">B</text><text x=\"128\" y=\"96\" fill=\"#6b6b76\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><text x=\"180\" y=\"96\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">142°</text><text x=\"20\" y=\"110\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">A</text><text x=\"172\" y=\"120\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">C</text></svg>",
              "diagramCaption": "The exterior angle at C is 142°; m∠B = 5x − 4. Not drawn to scale"
            },
            {
              "id": "ang-014",
              "topic": "Triangle Angle Relationships",
              "difficulty": "easy",
              "stem": "An isosceles triangle has an apex angle of 76°. What is the measure of one of its base angles?",
              "choices": [
                "52°",
                "76°",
                "104°",
                "38°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The interior angles sum to 180°, and the two base angles of an isosceles triangle are equal. So 76 + 2b = 180 gives 2b = 104 and b = 52°.",
                "76° repeats the apex angle. In an isosceles triangle it's the two base angles that are equal to each other, not equal to the apex.",
                "104° is the combined measure of both base angles. It still has to be halved, since the question asks for one of them.",
                "38° halves the apex angle instead of the remaining 104°. The apex isn't split by anything here."
              ],
              "hint": "Subtract the apex from 180° first — what's left belongs to two equal angles.",
              "diagram": "<svg viewBox=\"0 0 240 125\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"45\" y1=\"105\" x2=\"195\" y2=\"105\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"45\" y1=\"105\" x2=\"120\" y2=\"25\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"195\" y1=\"105\" x2=\"120\" y2=\"25\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"108\" y=\"48\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">76°</text><text x=\"56\" y=\"98\" fill=\"#6b6b76\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><text x=\"178\" y=\"98\" fill=\"#6b6b76\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text></svg>",
              "diagramCaption": "The two base angles are congruent. Not drawn to scale"
            },
            {
              "id": "ang-015",
              "topic": "Triangle Angle Relationships",
              "difficulty": "medium",
              "stem": "In a right triangle, the two acute angles differ by 28°. What is the measure of the larger acute angle?",
              "choices": [
                "31°",
                "59°",
                "62°",
                "74°"
              ],
              "correctIndex": 1,
              "explanations": [
                "31° is the smaller acute angle, 90 − 59. The question asks for the larger of the two.",
                "Correct. The acute angles of a right triangle sum to 90°. With x + y = 90 and x − y = 28, adding the equations gives 2x = 118, so x = 59°.",
                "62° comes from using 180° as the sum of the two acute angles instead of 90°: (180 + 28)/2 = 104 is too large, and 62° follows from a related slip. The right angle already uses 90° of the triangle's 180°.",
                "74° is (120 + 28)/2, or the result of splitting the difference against the wrong total. Check that the two acute angles add to 90°."
              ],
              "hint": "The right angle takes 90° of the triangle's 180°, so the two acute angles share what's left."
            },
            {
              "id": "ang-016",
              "topic": "Triangle Angle Relationships",
              "difficulty": "hard",
              "stem": "A dart (arrowhead) quadrilateral has three outer angles measuring 32°, 28°, and 35°. What is the measure of the reflex-forming interior angle x at the indented vertex?",
              "choices": [
                "63°",
                "95°",
                "85°",
                "72°"
              ],
              "correctIndex": 1,
              "explanations": [
                "63° adds only two of the three outer angles (28 + 35). All three contribute to the angle at the indented vertex.",
                "Correct. In a dart, the angle at the indented vertex equals the sum of the three remote outer angles: 32° + 28° + 35° = 95°. This follows from splitting the dart into two triangles with a line from the apex through the indented vertex.",
                "85° is the result of an arithmetic slip — 32 + 28 + 35 is 95, not 85. Re-add the three given angles.",
                "72° subtracts rather than adds somewhere, for example 180 − (32 + 28 + 35 + 13). The three outer angles combine additively here."
              ],
              "hint": "Draw a segment from the top vertex through the indented one. It splits the figure into two triangles, and the exterior angle relationship does the rest.",
              "diagram": "<svg viewBox=\"0 0 240 135\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><path d=\"M120,20 L50,120 L120,88 L190,120 Z\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><text x=\"126\" y=\"44\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">32°</text><text x=\"66\" y=\"112\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">28°</text><text x=\"150\" y=\"112\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">35°</text><text x=\"113\" y=\"80\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x</text></svg>",
              "diagramCaption": "A dart: the vertex marked x points inward. Not drawn to scale"
            },
            {
              "id": "ang-017",
              "topic": "Parallel Lines & Transversals",
              "difficulty": "hard",
              "stem": "Two parallel lines are cut by a transversal. One angle measures 4x² + 20 and its corresponding angle measures 20x + 4. Both solutions for x are positive — what is the greater one?",
              "choices": [
                "2",
                "4",
                "5",
                "8"
              ],
              "correctIndex": 1,
              "explanations": [
                "2 isn't a root. Substituting it gives 36 and 44, which aren't equal, so the corresponding-angle condition fails.",
                "Correct. Corresponding angles are congruent: 4x² + 20 = 20x + 4 rearranges to 4x² − 20x + 16 = 0, or x² − 5x + 4 = 0, which factors to (x − 4)(x − 1) = 0. The roots are 4 and 1, and the greater is 4.",
                "5 is the sum of the two roots (4 + 1), which appears as the middle coefficient of x² − 5x + 4. The sum of the roots isn't itself a root.",
                "8 comes from solving 4x² − 20x + 16 = 0 without dividing through by 4 first, then mis-factoring. Dividing by 4 gives the clean x² − 5x + 4."
              ],
              "hint": "Corresponding angles are congruent. Set the expressions equal, collect everything on one side, and divide out the common factor before factoring."
            },
            {
              "id": "ang-018",
              "topic": "Polygon Angle Sums",
              "difficulty": "easy",
              "stem": "What is the sum of the exterior angles of a convex nonagon, taking one exterior angle at each vertex?",
              "choices": [
                "360°",
                "540°",
                "1260°",
                "180°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The exterior angles of any convex polygon sum to 360°, no matter how many sides it has. Walking once around the polygon turns you through one complete revolution.",
                "540° is the interior angle sum of a pentagon, not an exterior sum for anything.",
                "1260° is the interior angle sum of a nonagon: (9 − 2) × 180. This is the interior total, and the question asks about exterior angles.",
                "180° is the interior angle sum of a triangle. Exterior angles complete a full turn, which is twice this."
              ],
              "hint": "Picture walking the perimeter once and returning to your starting heading. How far have you turned in total?"
            },
            {
              "id": "ang-019",
              "topic": "Angles Around a Point",
              "difficulty": "medium",
              "stem": "Three adjacent angles lie along a straight line. The second is double the first, and the third is triple the second. What is the measure of the second angle?",
              "choices": [
                "20°",
                "40°",
                "60°",
                "120°"
              ],
              "correctIndex": 1,
              "explanations": [
                "20° is the first angle, x. The question asks for the second, which is 2x.",
                "Correct. Let the first be x. The second is 2x and the third is 3(2x) = 6x. Along a straight line they sum to 180°, so 9x = 180 and x = 20. The second angle is 2(20) = 40°.",
                "60° would follow from reading the third angle as 3x rather than triple the second. 'Triple the second' means 3 × 2x = 6x, giving coefficients 1, 2, 6 rather than 1, 2, 3.",
                "120° is the third angle, 6x. It's the largest of the three, not the middle one."
              ],
              "hint": "Build the third angle from the second, not from the first — then use the straight-line total.",
              "diagram": "<svg viewBox=\"0 0 240 120\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"20\" y1=\"100\" x2=\"220\" y2=\"100\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"100\" x2=\"55\" y2=\"40\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"100\" x2=\"120\" y2=\"25\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"100\" x2=\"190\" y2=\"45\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"76\" y=\"92\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x</text><text x=\"110\" y=\"78\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">2x</text><text x=\"150\" y=\"88\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">6x</text><circle cx=\"120\" cy=\"100\" r=\"3\" fill=\"#7c3aed\"/></svg>",
              "diagramCaption": "Three adjacent angles on a straight line. Not drawn to scale"
            },
            {
              "id": "ang-020",
              "topic": "Parallel Lines & Transversals",
              "difficulty": "medium",
              "stem": "A horizontal cross-strut runs parallel to the base of a triangular frame. The base angle at the left is 58°. What is the measure of the corresponding angle x where the left side meets the strut?",
              "choices": [
                "58°",
                "122°",
                "62°",
                "42°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The strut is parallel to the base, and the left side of the frame is a transversal crossing both. Angle x and the 58° base angle are corresponding angles, so they're congruent: x = 58°.",
                "122° is the supplement of 58°. That would be the angle on the other side of the strut — the one forming a linear pair with x, not x itself.",
                "62° comes from treating the angles as complementary (90 − 58 = 32) or from an unrelated subtraction. Corresponding angles across parallel lines are equal, with no subtraction involved.",
                "42° doesn't follow from any relationship in the figure. Check which pair of angles the transversal actually creates."
              ],
              "hint": "The strut and the base are parallel, and one side of the frame crosses both. Identify which angle pair that makes.",
              "diagram": "<svg viewBox=\"0 0 240 130\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"110\" x2=\"215\" y2=\"110\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"110\" x2=\"120\" y2=\"25\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"215\" y1=\"110\" x2=\"120\" y2=\"25\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"68\" y1=\"72\" x2=\"172\" y2=\"72\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"40\" y=\"103\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">58°</text><text x=\"76\" y=\"66\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x</text></svg>",
              "diagramCaption": "The strut is parallel to the base. Not drawn to scale"
            },
            {
              "id": "ang-021",
              "topic": "Triangle Angle Relationships",
              "difficulty": "medium",
              "stem": "A triangle has angles 2x − 5, 3x + 10, and 4x − 5. How is the triangle classified by its angles?",
              "choices": [
                "Acute and right",
                "Right triangle",
                "Obtuse triangle",
                "Acute triangle"
              ],
              "correctIndex": 3,
              "explanations": [
                "No triangle can be both acute and right — the terms describe the same thing, the largest angle, and they contradict each other.",
                "Right would require one angle of exactly 90°. The angles here are 35°, 70°, and 75°; none is 90°.",
                "Obtuse would require an angle greater than 90°. The largest here is 75°.",
                "Correct. The angles sum to 180°: 9x = 180, so x = 20. That gives 35°, 70°, and 75°. Every angle is under 90°, so the triangle is acute."
              ],
              "hint": "Find the three actual measures first, then look only at the largest one — it decides the classification."
            },
            {
              "id": "ang-022",
              "topic": "Polygon Angle Sums",
              "difficulty": "easy",
              "stem": "What is the sum of the interior angles of a dodecagon (12 sides)?",
              "choices": [
                "1440°",
                "1800°",
                "2160°",
                "360°"
              ],
              "correctIndex": 1,
              "explanations": [
                "1440° uses n = 10: (10 − 2) × 180. A dodecagon has 12 sides, not 10.",
                "Correct. The interior angle sum is (n − 2) × 180. For n = 12 that's 10 × 180 = 1800°.",
                "2160° is 12 × 180, skipping the −2. The formula subtracts 2 because a polygon splits into n − 2 triangles, not n.",
                "360° is the sum of the exterior angles, which is 360° for every convex polygon. This question asks about interior angles."
              ],
              "hint": "A polygon with n sides divides into how many triangles? Multiply that count by 180°."
            },
            {
              "id": "ang-023",
              "topic": "Polygon Angle Sums",
              "difficulty": "easy",
              "stem": "Each exterior angle of a regular polygon measures 15°. How many vertices does it have?",
              "choices": [
                "12",
                "20",
                "24",
                "48"
              ],
              "correctIndex": 2,
              "explanations": [
                "12 would correspond to exterior angles of 30° (360 ÷ 12). Here each is 15°, which is half that, so the polygon has twice as many sides.",
                "20 would correspond to exterior angles of 18°.",
                "Correct. Exterior angles of a convex polygon sum to 360°, and in a regular polygon they're all equal. So n = 360 ÷ 15 = 24. A polygon has as many vertices as sides.",
                "48 comes from dividing 720 by 15. The exterior angles total 360°, not 720°."
              ],
              "hint": "The exterior angles are all equal and together make one full turn."
            },
            {
              "id": "ang-024",
              "topic": "Angle Pairs",
              "difficulty": "easy",
              "stem": "A vertical ray is perpendicular to a horizontal line at a point. Another ray from that same point makes a 38° angle with the horizontal line. What is the measure of the angle x between that ray and the vertical ray?",
              "choices": [
                "38°",
                "52°",
                "62°",
                "48°"
              ],
              "correctIndex": 1,
              "explanations": [
                "38° repeats the given angle. The two angles here are different parts of the right angle, not equal to each other.",
                "Correct. The vertical and horizontal rays form a 90° angle, and the two smaller angles split it. They're complementary: x = 90° − 38° = 52°.",
                "62° subtracts from 100 rather than 90. A perpendicular forms exactly 90°.",
                "48° subtracts from 86, or reflects an arithmetic slip. 90 − 38 = 52."
              ],
              "hint": "Perpendicular means exactly 90°. The two angles shown divide that right angle between them.",
              "diagram": "<svg viewBox=\"0 0 240 130\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"110\" x2=\"215\" y2=\"110\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"110\" x2=\"120\" y2=\"22\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><path d=\"M120,96 L134,96 L134,110\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><line x1=\"120\" y1=\"110\" x2=\"45\" y2=\"45\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"62\" y=\"100\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">38°</text><text x=\"96\" y=\"62\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x</text></svg>",
              "diagramCaption": "The vertical ray is perpendicular to the horizontal line. Not drawn to scale"
            },
            {
              "id": "ang-025",
              "topic": "Polygon Angle Sums",
              "difficulty": "medium",
              "stem": "The interior angle of a regular polygon is given by (8 × 180) ÷ 10. What is the measure of one exterior angle?",
              "choices": [
                "144°",
                "36°",
                "72°",
                "18°"
              ],
              "correctIndex": 1,
              "explanations": [
                "144° is the interior angle itself — the value the given expression evaluates to. The exterior angle is its supplement.",
                "Correct. The expression is (n − 2) × 180 ÷ n with n = 10, giving an interior angle of 144°. The exterior angle is the supplement: 180° − 144° = 36°.",
                "72° is double the exterior angle, or 360 ÷ 5. The polygon here has 10 sides, not 5.",
                "18° is 360 ÷ 20, which would describe a 20-sided polygon. The denominator 10 in the given expression is n."
              ],
              "hint": "Read n straight off the expression, then remember interior and exterior angles at a vertex form a straight line."
            },
            {
              "id": "ang-026",
              "topic": "Parallel Lines & Transversals",
              "difficulty": "medium",
              "stem": "Two parallel lines are cut by a transversal. A pair of alternate interior angles measure 8x − 40 and 4x + 16. What is the measure of each of these angles?",
              "choices": [
                "14°",
                "72°",
                "108°",
                "56°"
              ],
              "correctIndex": 1,
              "explanations": [
                "14° is the value of x, which still has to be substituted into one of the expressions.",
                "Correct. Alternate interior angles are congruent: 8x − 40 = 4x + 16 gives 4x = 56, so x = 14. Each angle measures 4(14) + 16 = 72°.",
                "108° is the supplement of 72°. That's the co-interior angle beside this pair, not the pair itself — alternate interior angles are equal, not supplementary.",
                "56° is 4x, a value that appears while solving but isn't an angle in the figure."
              ],
              "hint": "Alternate interior angles sit on opposite sides of the transversal. Decide whether that makes them equal or supplementary."
            },
            {
              "id": "ang-027",
              "topic": "Triangle Angle Relationships",
              "difficulty": "easy",
              "stem": "In a right triangle, one acute angle is four times the other. What is the measure of the smaller acute angle?",
              "choices": [
                "18°",
                "72°",
                "15°",
                "30°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The two acute angles sum to 90°, so x + 4x = 90 gives 5x = 90 and x = 18°.",
                "72° is the larger acute angle, 4x. The question asks for the smaller.",
                "15° comes from using 75° or 180° as the total in a way that mis-splits the ratio. The two acute angles of a right triangle share exactly 90°.",
                "30° would follow from x + 4x = 150, or from splitting 90° into thirds. The ratio 1:4 divides 90° into five parts, not three."
              ],
              "hint": "The right angle already accounts for 90°, so the two acute angles split the remaining 90° in a 1:4 ratio."
            },
            {
              "id": "ang-028",
              "topic": "Parallel Lines & Transversals",
              "difficulty": "easy",
              "stem": "Two parallel lines are cut by a transversal. One interior angle measures 112°. What is the measure of the co-interior (same-side interior) angle x?",
              "choices": [
                "112°",
                "68°",
                "78°",
                "88°"
              ],
              "correctIndex": 1,
              "explanations": [
                "112° would be correct for the alternate interior or corresponding angle, which are congruent to the given one. Same-side interior angles are supplementary instead.",
                "Correct. Co-interior angles between parallel lines are supplementary: x = 180° − 112° = 68°.",
                "78° subtracts from 190 rather than 180. A straight line is 180°.",
                "88° subtracts from 200, or reflects an arithmetic slip. 180 − 112 = 68."
              ],
              "hint": "Same-side interior angles are on the same side of the transversal. That side relationship makes them supplementary, not equal.",
              "diagram": "<svg viewBox=\"0 0 240 132\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"15\" y1=\"30\" x2=\"225\" y2=\"30\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"15\" y1=\"110\" x2=\"225\" y2=\"110\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"50\" y1=\"20\" x2=\"165\" y2=\"120\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"72\" y=\"50\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">112°</text><text x=\"112\" y=\"100\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x</text></svg>",
              "diagramCaption": "The two horizontal lines are parallel. Not drawn to scale"
            },
            {
              "id": "ang-029",
              "topic": "Polygon Angle Sums",
              "difficulty": "medium",
              "stem": "The interior angles of a convex polygon sum to 900°. How many sides does it have?",
              "choices": [
                "5",
                "6",
                "7",
                "8"
              ],
              "correctIndex": 2,
              "explanations": [
                "5 gives (5 − 2) × 180 = 540°, the pentagon's interior sum.",
                "6 gives (6 − 2) × 180 = 720°, the hexagon's interior sum.",
                "Correct. Set (n − 2) × 180 = 900. Dividing gives n − 2 = 5, so n = 7 — a heptagon.",
                "8 gives (8 − 2) × 180 = 1080°, which overshoots 900°."
              ],
              "hint": "Divide the total by 180° first — what's left is the number of triangles the polygon splits into, which is two fewer than the number of sides."
            },
            {
              "id": "ang-030",
              "topic": "Angles Around a Point",
              "difficulty": "medium",
              "stem": "Four rays from a common point divide the plane into four angles measuring x, 2x + 10, 3x − 20, and 4x + 10. What is the measure of the largest angle?",
              "choices": [
                "36°",
                "82°",
                "124°",
                "154°"
              ],
              "correctIndex": 3,
              "explanations": [
                "36° is the value of x, which is also the smallest angle here — but it's the value of the variable, and the question asks for the largest angle.",
                "82° is 2x + 10, the second angle. Compare all four measures before choosing.",
                "124° is 3x − 20, the third angle. The largest coefficient belongs to 4x + 10.",
                "Correct. A full turn is 360°. Summing gives x + (2x + 10) + (3x − 20) + (4x + 10) = 10x = 360, so x = 36. The largest angle is 4(36) + 10 = 154°."
              ],
              "hint": "The constants cancel when you add all four expressions. Angles filling a complete turn sum to 360°.",
              "diagram": "<svg viewBox=\"0 0 240 150\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><circle cx=\"120\" cy=\"72\" r=\"3\" fill=\"#7c3aed\"/><line x1=\"120\" y1=\"72\" x2=\"225\" y2=\"72\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"72\" x2=\"165\" y2=\"15\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"72\" x2=\"35\" y2=\"40\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"72\" x2=\"70\" y2=\"135\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><text x=\"160\" y=\"58\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x</text><text x=\"104\" y=\"30\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">2x + 10</text><text x=\"30\" y=\"72\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3x − 20</text><text x=\"122\" y=\"126\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4x + 10</text></svg>",
              "diagramCaption": "Four angles filling a complete turn. Not drawn to scale"
            },
            {
              "id": "ang-031",
              "topic": "Complementary Angles",
              "difficulty": "medium",
              "stem": "Two angles are complementary and one is 27° larger than the other. Find the smaller angle.",
              "choices": [
                "31.5°",
                "58.5°",
                "76.5°",
                "63°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Complementary means the two sum to 90°. Let the smaller be x; then x + (x + 27) = 90, so 2x = 63 and x = 31.5°. The larger is 58.5°, and 31.5 + 58.5 = 90 as required.",
                "58.5° is the LARGER of the two angles, not the smaller. It is the correct pair, but the question asks for the other one.",
                "76.5° comes from treating the angles as supplementary: (180 − 27)/2 = 76.5. Complementary is 90°, not 180° — this is the single most common slip on this problem type.",
                "63° is 90 − 27, which subtracts the difference from the total in one step. That skips the fact that the difference has to be split between the two angles, not removed entirely."
              ],
              "hint": "Complementary fixes the sum. Name the smaller angle x and write the larger in terms of it."
            },
            {
              "id": "ang-032",
              "topic": "Supplement and Complement",
              "difficulty": "hard",
              "stem": "An angle's supplement is 4 times its complement. Find the angle.",
              "choices": [
                "30°",
                "45°",
                "60°",
                "72°"
              ],
              "correctIndex": 2,
              "explanations": [
                "30° gives a supplement of 150° and a complement of 60°. But 4 × 60 = 240, not 150, so the condition fails.",
                "45° gives a supplement of 135° and a complement of 45°. Then 4 × 45 = 180, not 135. This choice is tempting because 45° is self-complementary, but that is not what the problem asks.",
                "Correct. Let the angle be a. Its supplement is 180 − a and its complement is 90 − a, so 180 − a = 4(90 − a) = 360 − 4a. Then 3a = 180 and a = 60°. Check: supplement 120°, complement 30°, and 120 = 4 × 30.",
                "72° gives a supplement of 108° and a complement of 18°. Then 4 × 18 = 72, which is the angle itself rather than the supplement — a sign the two expressions got swapped."
              ],
              "hint": "Write both the supplement and the complement as expressions in the same unknown before setting up the equation."
            },
            {
              "id": "ang-033",
              "topic": "Co-interior Angles",
              "difficulty": "medium",
              "stem": "Lines ℓ ∥ m are cut by a transversal. A co-interior (same-side interior) pair measures (2x + 10)° and (3x − 30)°. Find x.",
              "choices": [
                "20",
                "40",
                "8",
                "44"
              ],
              "correctIndex": 1,
              "explanations": [
                "x = 20 comes from setting the two expressions EQUAL: 2x + 10 = 3x − 30 gives x = 40, not 20 — but students who mis-solve that equation often land here. Co-interior angles are not equal in the first place.",
                "Correct. Co-interior angles on the same side of a transversal between parallel lines are SUPPLEMENTARY. So (2x + 10) + (3x − 30) = 180, giving 5x − 20 = 180, then 5x = 200 and x = 40. Both angles work out to 90°.",
                "x = 8 comes from setting the sum to 90° instead of 180°. Co-interior angles are supplementary, not complementary — the pair straddles a straight line, not a right angle.",
                "x = 44 comes from solving 5x = 220, which drops the sign on the −20 when moving it across. The constant is −20, so it is added to 180, not subtracted."
              ],
              "hint": "Of the four transversal relationships, three give equality and one gives a sum of 180°. Identify which one same-side interior angles fall under."
            },
            {
              "id": "ang-034",
              "topic": "Vertical Angles",
              "difficulty": "easy",
              "stem": "Two vertical angles measure (5y − 12)° and (3y + 20)°. Find the measure of each angle.",
              "choices": [
                "16°",
                "68°",
                "112°",
                "84°"
              ],
              "correctIndex": 1,
              "explanations": [
                "16° is the value of y, not the angle. Solving 5y − 12 = 3y + 20 gives 2y = 32 and y = 16, but the question asks for the angle measure, which still needs substituting back.",
                "Correct. Vertical angles are equal, so 5y − 12 = 3y + 20, giving 2y = 32 and y = 16. Substituting back: 5(16) − 12 = 68°, and the check 3(16) + 20 = 68° agrees.",
                "112° is the SUPPLEMENT of the correct answer. It comes from treating the two angles as a linear pair summing to 180° rather than as vertical angles, which are equal.",
                "84° comes from substituting y = 16 into an incorrectly simplified expression. Both given expressions must evaluate to the same number — check one against the other before committing."
              ],
              "hint": "Vertical angles are equal. Solve for the variable first, then remember the question asks for the angle, not the variable."
            },
            {
              "id": "ang-035",
              "topic": "Angles on a Line",
              "difficulty": "medium",
              "stem": "Three angles lie along a straight line and measure x°, (2x)°, and (x + 40)°. Find the largest of the three.",
              "choices": [
                "35°",
                "70°",
                "75°",
                "105°"
              ],
              "correctIndex": 2,
              "explanations": [
                "35° is the value of x and the SMALLEST of the three angles. Solving x + 2x + (x + 40) = 180 gives 4x = 140 and x = 35, but that is only the first angle.",
                "70° is the middle angle, 2x = 2(35). Comparing all three — 35°, 70° and 75° — shows this is not the largest.",
                "Correct. The three angles form a straight angle, so x + 2x + (x + 40) = 180, giving 4x + 40 = 180, then 4x = 140 and x = 35. The three angles are 35°, 70° and 75°, so the largest is 75°.",
                "105° would come from using 360° as the total instead of 180°. Angles along a straight LINE sum to 180°; it is angles around a POINT that sum to 360°."
              ],
              "hint": "Angles that together form a straight line have a fixed sum. Find that sum first, then compare all three results."
            },
            {
              "id": "ang-036",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "Two parallel lines are cut by a transversal. A student sees a pair of co-interior (same-side interior) angles and marks them congruent. What is the correct relationship, and why?",
              "choices": [
                "They are congruent, because all angles formed by parallel lines are congruent.",
                "They are supplementary, summing to 180°, because they form a linear pair with a pair of congruent alternate interior angles.",
                "They are complementary, summing to 90°.",
                "There is no fixed relationship without knowing the angle of the transversal."
              ],
              "correctIndex": 1,
              "explanations": [
                "Only some of the pairs are congruent — corresponding, alternate interior and alternate exterior. Co-interior angles are the pair that is supplementary instead.",
                "Correct. Same-side interior angles sum to 180°. Congruence holds for corresponding and alternate pairs, but not for this one.",
                "Complementary means 90°, which does not arise from parallel lines and a transversal.",
                "The relationship is fixed by the lines being parallel, whatever angle the transversal makes."
              ],
              "hint": "Sort the transversal pairs into the ones that are congruent and the one that is supplementary.",
              "kind": "reasoning"
            },
            {
              "id": "ang-037",
              "topic": "Reasoning: What's Missing",
              "difficulty": "hard",
              "stem": "A diagram shows two lines cut by a transversal, with one pair of corresponding angles measuring 63° and 63°. What can be concluded, and by which reasoning?",
              "choices": [
                "Nothing — congruent corresponding angles can occur without the lines being parallel.",
                "The lines are parallel, by the converse of the Corresponding Angles Postulate.",
                "The lines are perpendicular, since 63° is not 90°.",
                "The lines must be the same line."
              ],
              "correctIndex": 1,
              "explanations": [
                "This gets the logic backwards. The CONVERSE is a theorem in its own right: congruent corresponding angles force the lines to be parallel.",
                "Correct. If two lines are cut by a transversal and corresponding angles are congruent, the lines are parallel. Using a theorem in this direction is what a converse is for.",
                "Perpendicular lines meet at 90°. Nothing here indicates that, and 63° would rule it out.",
                "Two distinct parallel lines are not the same line — they simply never meet."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "geometry-starter",
          "title": "Foundations: Starter Quiz",
          "blurb": "Special right triangles, circle theorems, coordinate transformations, and quadrilateral properties. Five questions include diagrams.",
          "meta": "22 questions · easy → hard",
          "categories": [
            "triangles-review",
            "properties-of-triangles",
            "circles",
            "congruence-transformations",
            "quadrilaterals"
          ],
          "questions": [
            {
              "id": "tri-001",
              "topic": "Special Right Triangles & Triangle Classification",
              "difficulty": "easy",
              "stem": "In a 45-45-90 triangle, the legs each measure 7 units. What is the length of the hypotenuse?",
              "choices": [
                "7 units",
                "7√2 units",
                "14 units",
                "7√3 units"
              ],
              "correctIndex": 1,
              "explanations": [
                "This is just the leg length restated — the hypotenuse of a 45-45-90 triangle is always longer than either leg.",
                "In a 45-45-90 triangle, hypotenuse = leg × √2 = 7√2 units.",
                "Doubling the leg overstates the hypotenuse — that relationship doesn't apply here since the legs are equal.",
                "√3 relates the two legs of a 30-60-90 triangle, not the leg-to-hypotenuse ratio of a 45-45-90 triangle."
              ]
            },
            {
              "id": "tri-002",
              "topic": "Special Right Triangles & Triangle Classification",
              "difficulty": "easy",
              "stem": "In a 30-60-90 triangle, the side opposite the 30° angle is 5 units. What is the length of the side opposite the 60° angle?",
              "choices": [
                "5 units",
                "10 units",
                "5√2 units",
                "5√3 units"
              ],
              "correctIndex": 3,
              "explanations": [
                "5 units is the short leg itself (opposite 30°) — the side opposite 60° is longer than the short leg.",
                "10 units would be the hypotenuse (opposite the 90° angle), which is twice the short leg — not the side opposite 60°.",
                "√2 relates legs in a 45-45-90 triangle, not the short-to-long leg ratio in a 30-60-90 triangle.",
                "In a 30-60-90 triangle, the side opposite 60° equals the short leg times √3: 5√3 units."
              ]
            },
            {
              "id": "tri-003",
              "topic": "Special Right Triangles & Triangle Classification",
              "difficulty": "medium",
              "stem": "Which set of side lengths could NOT form a valid triangle?",
              "choices": [
                "7, 10, 15",
                "10, 10, 10",
                "5, 8, 14",
                "3, 4, 5"
              ],
              "correctIndex": 2,
              "explanations": [
                "7 + 10 = 17 > 15, so this satisfies the Triangle Inequality Theorem and forms a valid triangle.",
                "An equilateral triangle always satisfies the Triangle Inequality Theorem, since all sides are equal.",
                "5 + 8 = 13, which is less than 14. The sum of the two shorter sides must exceed the longest side, so these lengths cannot close into a triangle.",
                "This is the classic 3-4-5 right triangle; 3 + 4 = 7 > 5, so it's valid."
              ],
              "hint": "Add the two shorter sides. If that sum isn't greater than the longest side, the triangle can't close."
            },
            {
              "id": "tri-004",
              "topic": "Special Right Triangles & Triangle Classification",
              "difficulty": "medium",
              "stem": "A triangle has sides of length 9, 12, and 15. Is this triangle a right triangle?",
              "choices": [
                "Yes, because 9 + 12 = 21 ≠ 15²",
                "Yes, because 9² + 12² = 15²",
                "No, because the angles don't sum to 180°",
                "No, because 9, 12, and 15 aren't a Pythagorean triple"
              ],
              "correctIndex": 1,
              "explanations": [
                "This compares the wrong quantities — the Converse of the Pythagorean Theorem checks a² + b² against c², not a simple sum against c².",
                "9² + 12² = 81 + 144 = 225, and 15² = 225. Since the sum of the squares of the two shorter sides equals the square of the longest side, the Converse of the Pythagorean Theorem confirms this is a right triangle.",
                "Every triangle's interior angles sum to 180°, regardless of whether it's a right triangle — this isn't a valid test.",
                "9-12-15 is actually a Pythagorean triple — it's 3-4-5 scaled by 3."
              ],
              "hint": "Use the Converse of the Pythagorean Theorem: compare (short leg)² + (other leg)² to (longest side)².",
              "diagram": "<svg viewBox='0 0 240 200' xmlns='http://www.w3.org/2000/svg'><polygon points='30,170 225,150 70,25' fill='none' stroke='#170f22' stroke-width='2'/><text x='108' y='178' font-size='14' fill='#6d28d9'>12</text><text x='155' y='92' font-size='14' fill='#6d28d9'>15</text><text x='8' y='102' font-size='14' fill='#6d28d9'>9</text></svg>"
            },
            {
              "id": "tri-005",
              "topic": "Special Right Triangles & Triangle Classification",
              "difficulty": "hard",
              "stem": "A right triangle has legs of length 8 and 15. What is the length of the altitude drawn from the right angle to the hypotenuse?",
              "choices": [
                "7.5 units",
                "120/17 units",
                "23 units",
                "17/2 units"
              ],
              "correctIndex": 1,
              "explanations": [
                "7.5 is half of 15 — it doesn't come from the altitude-on-hypotenuse relationship.",
                "For a right triangle, the altitude to the hypotenuse equals (leg₁ × leg₂) / hypotenuse. With legs 8 and 15, the hypotenuse is √(8²+15²) = 17, so the altitude is (8×15)/17 = 120/17 units.",
                "23 is just 8 + 15 — adding the legs doesn't correspond to any length in this triangle.",
                "17/2 is half the hypotenuse — that's the median to the hypotenuse, not the altitude; they only coincide in an isosceles right triangle."
              ],
              "hint": "First find the hypotenuse with the Pythagorean Theorem, then use: altitude = (leg₁ × leg₂) ÷ hypotenuse."
            },
            {
              "id": "cir-001",
              "topic": "Circle Theorems",
              "difficulty": "easy",
              "stem": "A central angle in a circle measures 70°. What is the measure of the arc it intercepts?",
              "choices": [
                "35°",
                "70°",
                "140°",
                "180°"
              ],
              "correctIndex": 1,
              "explanations": [
                "35° is half of 70° — that halving relationship applies to inscribed angles, not central angles.",
                "A central angle and its intercepted arc always have the same measure: 70°.",
                "140° would be double the central angle — that's not how a central angle relates to its own arc.",
                "180° is a semicircle; nothing here indicates this arc spans half the circle."
              ]
            },
            {
              "id": "cir-002",
              "topic": "Circle Theorems",
              "difficulty": "medium",
              "stem": "An inscribed angle intercepts an arc measuring 88°. What is the measure of the inscribed angle?",
              "choices": [
                "44°",
                "88°",
                "176°",
                "92°"
              ],
              "correctIndex": 0,
              "explanations": [
                "The Inscribed Angle Theorem states an inscribed angle is always half its intercepted arc: 88° ÷ 2 = 44°.",
                "88° is the arc measure itself (equal to the central angle) — not the inscribed angle.",
                "176° would be double the arc, moving in the wrong direction from the halving relationship.",
                "92° isn't derived from the 88° arc through any standard circle relationship here."
              ],
              "hint": "Inscribed angle = half of its intercepted arc."
            },
            {
              "id": "cir-003",
              "topic": "Circle Theorems",
              "difficulty": "medium",
              "stem": "Two chords intersect inside a circle, forming an angle of 55°. If one of the intercepted arcs measures 70°, what is the measure of the other intercepted arc?",
              "choices": [
                "15°",
                "40°",
                "70°",
                "110°"
              ],
              "correctIndex": 1,
              "explanations": [
                "15° comes from subtracting the angle from the given arc, which isn't how the two-chords relationship works.",
                "When two chords intersect inside a circle, the angle formed equals half the sum of the two intercepted arcs: 55 = ½(70 + x), so 110 = 70 + x, giving x = 40°.",
                "70° just repeats the given arc — the two intercepted arcs aren't required to be equal.",
                "110° is the sum of both arcs (2 × 55°), not the second arc alone — you still need to subtract the known arc."
              ],
              "hint": "Angle = ½ (arc₁ + arc₂). Set up the equation and solve for the missing arc.",
              "diagram": "<svg viewBox='0 0 220 225' xmlns='http://www.w3.org/2000/svg'><circle cx='110' cy='110' r='80' fill='none' stroke='#170f22' stroke-width='2'/><line x1='183' y1='76' x2='35' y2='137' stroke='#170f22' stroke-width='1.5'/><line x1='83' y1='35' x2='171' y2='161' stroke='#170f22' stroke-width='1.5'/><polyline points='148,91 142,82 134,78 124,77 115,81' fill='none' stroke='#6d28d9' stroke-width='1.5'/><polyline points='35,137 59,171 96,189 137,185 171,161' fill='none' stroke='#6d28d9' stroke-width='2.5'/><polyline points='183,76 161,49 131,33 96,31 83,35' fill='none' stroke='#6b6b76' stroke-width='2.5'/><text x='114' y='63' font-size='13' fill='#6d28d9'>55°</text><text x='75' y='207' font-size='13' fill='#6d28d9'>70°</text><text x='135' y='22' font-size='13' fill='#6b6b76'>?</text></svg>"
            },
            {
              "id": "cir-004",
              "topic": "Circle Theorems",
              "difficulty": "hard",
              "stem": "From an external point, two secant segments are drawn to a circle. One secant has an external segment of 4 and a total length of 10. The other secant has an external segment of 5. What is the total length of the second secant?",
              "choices": [
                "6",
                "7",
                "8",
                "9"
              ],
              "correctIndex": 2,
              "explanations": [
                "6 doesn't satisfy the Power of a Point equation for these two secants.",
                "7 is close but doesn't balance the products — recheck: 4×10 must equal 5×total₂.",
                "By the Power of a Point theorem, the products of each secant's external segment and total length are equal: 4 × 10 = 5 × total₂, so total₂ = 40/5 = 8.",
                "9 overshoots — the product 5×9 = 45 doesn't match 4×10 = 40."
              ],
              "hint": "Power of a Point: (external segment) × (whole secant) is equal for both secants from the same external point.",
              "diagram": "<svg viewBox='0 0 260 220' xmlns='http://www.w3.org/2000/svg'><circle cx='150' cy='110' r='65' fill='none' stroke='#170f22' stroke-width='2'/><line x1='20' y1='190' x2='230' y2='70' stroke='#170f22' stroke-width='1.5'/><line x1='20' y1='190' x2='150' y2='30' stroke='#170f22' stroke-width='1.5'/><line x1='93' y1='142' x2='99' y2='152' stroke='#170f22' stroke-width='1.5'/><line x1='80' y1='106' x2='90' y2='114' stroke='#170f22' stroke-width='1.5'/><circle cx='20' cy='190' r='3' fill='#170f22'/><text x='4' y='206' font-size='12' fill='#170f22'>P</text><text x='46' y='167' font-size='13' fill='#6d28d9'>4</text><text x='160' y='98' font-size='13' fill='#6d28d9'>10</text><text x='40' y='150' font-size='13' fill='#6d28d9'>5</text><text x='128' y='58' font-size='13' fill='#6b6b76'>?</text></svg>"
            },
            {
              "id": "trf-001",
              "topic": "Coordinate Transformations",
              "difficulty": "easy",
              "stem": "Which transformation rule represents a 180° rotation about the origin?",
              "choices": [
                "(x,y) → (y,−x)",
                "(x,y) → (−y,x)",
                "(x,y) → (−x,−y)",
                "(x,y) → (x,−y)"
              ],
              "correctIndex": 2,
              "explanations": [
                "This is a 90° clockwise rotation about the origin.",
                "This is a 90° counter-clockwise rotation about the origin.",
                "A 180° rotation about the origin negates both coordinates: (x,y) → (−x,−y).",
                "This is a reflection across the x-axis, not a rotation."
              ]
            },
            {
              "id": "trf-002",
              "topic": "Coordinate Transformations",
              "difficulty": "easy",
              "stem": "A point (3,−2) is reflected across the y-axis. What are the coordinates of the image?",
              "choices": [
                "(3,2)",
                "(−3,−2)",
                "(−3,2)",
                "(2,−3)"
              ],
              "correctIndex": 1,
              "explanations": [
                "This reflects across the x-axis, not the y-axis — the y-coordinate's sign flips instead of the x-coordinate's.",
                "Reflecting across the y-axis negates the x-coordinate and keeps the y-coordinate: (3,−2) → (−3,−2).",
                "This flips both coordinates' signs, which is a 180° rotation, not a reflection across the y-axis.",
                "This swaps the coordinates, which relates to reflection across the line y = x, not the y-axis."
              ]
            },
            {
              "id": "trf-003",
              "topic": "Coordinate Transformations",
              "difficulty": "medium",
              "stem": "A 2D shape with area 20 sq units is dilated by a scale factor k = 3. What is the area of the dilated shape?",
              "choices": [
                "120 sq units",
                "60 sq units",
                "180 sq units",
                "540 sq units"
              ],
              "correctIndex": 2,
              "explanations": [
                "120 doesn't correspond to squaring the scale factor — check how area scales under dilation.",
                "60 = 20 × 3 scales area the same way as length, but area scales by the square of the scale factor, not the factor itself.",
                "Area scales by the square of the scale factor: 20 × 3² = 20 × 9 = 180 sq units.",
                "540 = 20 × 27, which would be the effect of cubing the scale factor — that relationship applies to volume, not area."
              ],
              "hint": "Under a dilation by factor k, length scales by k, area scales by k², and volume scales by k³."
            },
            {
              "id": "trf-004",
              "topic": "Coordinate Transformations",
              "difficulty": "hard",
              "stem": "Triangle ABC has vertices A(0,0), B(4,0), C(0,3). It is rotated 90° counter-clockwise about the origin, then translated 2 units right and 1 unit up. What are the final coordinates of the image of point B?",
              "choices": [
                "(2,5)",
                "(0,4)",
                "(6,1)",
                "(−2,3)"
              ],
              "correctIndex": 0,
              "explanations": [
                "Rotating B(4,0) by 90° CCW about the origin gives (−y,x) = (0,4). Translating that 2 right and 1 up gives (0+2, 4+1) = (2,5).",
                "(0,4) is the point after rotation only — the translation step still needs to be applied.",
                "(6,1) doesn't match the rotation formula or the translation applied correctly — double-check the rotation rule (x,y) → (−y,x).",
                "(−2,3) suggests using the 90° clockwise rotation rule (x,y) → (y,−x) instead of counter-clockwise."
              ],
              "hint": "Do the transformations in order: rotate first using (x,y)→(−y,x), then translate the result.",
              "diagram": "<svg viewBox='0 0 220 220' xmlns='http://www.w3.org/2000/svg'><line x1='10' y1='180' x2='210' y2='180' stroke='#c7c5d6' stroke-width='1'/><line x1='40' y1='200' x2='40' y2='10' stroke='#c7c5d6' stroke-width='1'/><polygon points='40,180 160,180 40,90' fill='none' stroke='#170f22' stroke-width='2'/><circle cx='40' cy='180' r='3' fill='#6d28d9'/><circle cx='160' cy='180' r='3' fill='#6d28d9'/><circle cx='40' cy='90' r='3' fill='#6d28d9'/><text x='18' y='196' font-size='12' fill='#170f22'>A(0,0)</text><text x='150' y='196' font-size='12' fill='#170f22'>B(4,0)</text><text x='6' y='86' font-size='12' fill='#170f22'>C(0,3)</text></svg>",
              "diagramCaption": "Triangle ABC before the rotation and translation"
            },
            {
              "id": "qua-001",
              "topic": "Quadrilateral Properties",
              "difficulty": "easy",
              "stem": "Which quadrilateral is guaranteed to have diagonals that are both equal in length AND perpendicular to each other?",
              "choices": [
                "Rectangle",
                "Rhombus",
                "Square",
                "Parallelogram"
              ],
              "correctIndex": 2,
              "explanations": [
                "A rectangle's diagonals are always equal in length, but they're only perpendicular if the rectangle is also a square.",
                "A rhombus's diagonals are always perpendicular, but they're only equal in length if the rhombus is also a square.",
                "A square inherits both properties — it's a rectangle (equal diagonals) and a rhombus (perpendicular diagonals) at once.",
                "A general parallelogram's diagonals bisect each other but are neither guaranteed equal nor perpendicular."
              ]
            },
            {
              "id": "qua-002",
              "topic": "Quadrilateral Properties",
              "difficulty": "medium",
              "stem": "In parallelogram ABCD, diagonals AC and BD intersect at point E. If AE = 3x−1 and EC = x+7, what is the value of x?",
              "choices": [
                "2",
                "3",
                "4",
                "6"
              ],
              "correctIndex": 2,
              "explanations": [
                "x = 2 doesn't satisfy 3x−1 = x+7 — recheck the algebra after setting the two segments equal.",
                "x = 3 gives AE = 8 and EC = 10, which aren't equal — the diagonals must bisect each other in a parallelogram.",
                "Since a parallelogram's diagonals bisect each other, AE = EC: 3x−1 = x+7, so 2x = 8 and x = 4.",
                "x = 6 overshoots; plugging back in gives AE = 17 and EC = 13, which don't match."
              ],
              "hint": "A parallelogram's diagonals bisect each other, so the two halves of each diagonal are equal.",
              "diagram": "<svg viewBox='0 0 280 190' xmlns='http://www.w3.org/2000/svg'><polygon points='30,160 220,160 260,40 70,40' fill='none' stroke='#170f22' stroke-width='2'/><line x1='30' y1='160' x2='260' y2='40' stroke='#170f22' stroke-width='1.5'/><line x1='220' y1='160' x2='70' y2='40' stroke='#170f22' stroke-width='1.5'/><circle cx='145' cy='100' r='3' fill='#170f22'/><text x='14' y='178' font-size='13' fill='#170f22'>A</text><text x='222' y='178' font-size='13' fill='#170f22'>B</text><text x='262' y='34' font-size='13' fill='#170f22'>C</text><text x='54' y='34' font-size='13' fill='#170f22'>D</text><text x='150' y='94' font-size='13' fill='#170f22'>E</text><text x='58' y='142' font-size='13' fill='#6d28d9'>3x−1</text><text x='185' y='64' font-size='13' fill='#6d28d9'>x+7</text></svg>"
            },
            {
              "id": "qua-003",
              "topic": "Quadrilateral Properties",
              "difficulty": "medium",
              "stem": "A trapezoid has parallel sides of length 8 and 14, and a height of 5. What is its area?",
              "choices": [
                "55 sq units",
                "110 sq units",
                "35 sq units",
                "70 sq units"
              ],
              "correctIndex": 0,
              "explanations": [
                "The trapezoid area formula is ½(b₁+b₂)h = ½(8+14)(5) = ½(22)(5) = 55 sq units.",
                "110 skips the ½ factor in the trapezoid area formula.",
                "35 comes from using only one base (14) with the height and halving — both parallel sides must be included.",
                "70 = 14 × 5 treats this like a rectangle or parallelogram, ignoring that the top side is shorter than the bottom."
              ]
            },
            {
              "id": "qua-004",
              "topic": "Quadrilateral Properties",
              "difficulty": "hard",
              "stem": "Quadrilateral PQRS has vertices P(0,0), Q(6,0), R(6,4), S(0,4). A student claims PQRS must be a square because it has four right angles. Is the student correct?",
              "choices": [
                "Yes, four right angles always guarantee a square",
                "No, PQRS is a rectangle, not a square, because its side lengths aren't all equal",
                "No, PQRS isn't even a parallelogram",
                "Yes, because opposite sides are parallel"
              ],
              "correctIndex": 1,
              "explanations": [
                "Four right angles guarantee a rectangle, but a square additionally requires all four sides to be equal — that extra condition isn't automatic.",
                "PQ = 6 and QR = 4, so adjacent sides aren't equal. PQRS has four right angles (a rectangle) but fails the equal-sides requirement for a square.",
                "PQRS does satisfy parallelogram conditions (opposite sides parallel and equal: PQ‖SR = 6, QR‖PS = 4) — the issue is specifically the square claim, not the parallelogram claim.",
                "Opposite sides being parallel confirms it's a parallelogram (and here, a rectangle), but doesn't address whether all sides are equal, which is what 'square' requires."
              ],
              "hint": "Plot the points or compute PQ and QR directly. A square needs four right angles AND four equal sides."
            },
            {
              "id": "tri-006",
              "topic": "Triangle Inequality",
              "difficulty": "easy",
              "stem": "Two sides of a triangle measure 5 and 8. Which range describes all possible lengths for the third side?",
              "choices": [
                "3 < s < 13",
                "5 < s < 8",
                "0 < s < 13",
                "3 < s < 40"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The Triangle Inequality requires any two sides to sum to more than the third. So s < 5 + 8 = 13 and s > 8 − 5 = 3. The third side must lie strictly between the difference and the sum.",
                "This confuses the range of the third side with the range between the two given sides. Nothing requires the third side to fall between 5 and 8 — a triangle with sides 5, 8 and 4 is perfectly valid.",
                "The upper bound is right but the lower bound is missing. A side of length 1 with sides 5 and 8 fails, since 1 + 5 = 6 which is less than 8 — the two shorter sides could not reach across.",
                "This multiplies the given sides rather than adding them. The Triangle Inequality is about sums and differences of side lengths, never products."
              ],
              "hint": "There are two constraints, not one: the third side must be short enough and also long enough."
            },
            {
              "id": "tri-007",
              "topic": "Exterior Angle Theorem",
              "difficulty": "medium",
              "stem": "An exterior angle of a triangle measures 110°, and one of its remote interior angles measures 45°. Find the other remote interior angle.",
              "choices": [
                "25°",
                "65°",
                "70°",
                "135°"
              ],
              "correctIndex": 1,
              "explanations": [
                "25° comes from 70 − 45, which first converts the exterior angle to its adjacent interior angle and then subtracts. The Exterior Angle Theorem uses the exterior angle directly, with no conversion step.",
                "Correct. The Exterior Angle Theorem says an exterior angle equals the SUM of the two remote interior angles. So 110 = 45 + x, giving x = 65°.",
                "70° is the interior angle adjacent to the exterior angle (180 − 110). That is the third angle of the triangle, not one of the two remote ones.",
                "135° comes from 180 − 45, which treats the 45° angle as part of a linear pair. The 45° angle is a remote interior angle, so it is not adjacent to the exterior angle at all."
              ],
              "hint": "The exterior angle equals the sum of the two angles it does not touch. Set up an equation from that directly."
            },
            {
              "id": "cir-005",
              "topic": "Arc Length",
              "difficulty": "medium",
              "stem": "Find the length of a 90° arc in a circle of radius 8.",
              "choices": [
                "2π",
                "4π",
                "16π",
                "8π"
              ],
              "correctIndex": 1,
              "explanations": [
                "2π would be a quarter of the RADIUS-based circumference 2πr with r mistakenly taken as 4, or a quarter of 8π. Recompute the full circumference before taking the fraction.",
                "Correct. Arc length = (θ/360) · 2πr = (90/360) · 2π(8) = ¼ · 16π = 4π. The fraction of the circle comes first, then it multiplies the full circumference.",
                "16π is the FULL circumference of the circle, 2π(8). The 90° portion is only a quarter of it, so the fraction step was skipped.",
                "8π is half the circumference, which would correspond to a 180° arc. A 90° arc is a quarter, not a half."
              ],
              "hint": "An arc is a fraction of the whole circumference. Work out the fraction from the angle, then apply it."
            },
            {
              "id": "trf-005",
              "topic": "Scale and Area",
              "difficulty": "medium",
              "stem": "A figure has area 20. After a dilation by scale factor k = 3, what is the new area?",
              "choices": [
                "60",
                "180",
                "23",
                "540"
              ],
              "correctIndex": 1,
              "explanations": [
                "60 is 20 × 3, which scales the area by k rather than k². Lengths scale by k, but area is a two-dimensional measure and picks up the factor twice.",
                "Correct. Under a dilation by k, area scales by k². So the new area is 20 × 3² = 20 × 9 = 180. This is one of the highest-risk facts in the whole transformations topic.",
                "23 adds the scale factor to the area instead of multiplying. Dilation is a multiplicative operation on measurements, never additive.",
                "540 is 20 × 3³, which uses the VOLUME scale factor. k³ applies to three-dimensional solids; a plane figure's area uses k²."
              ],
              "hint": "Ask how many dimensions the measurement has. That number is the exponent on the scale factor."
            },
            {
              "id": "qua-005",
              "topic": "Rhombus Diagonals",
              "difficulty": "medium",
              "stem": "A rhombus has diagonals of length 6 and 8. Find the length of one side.",
              "choices": [
                "5",
                "7",
                "10",
                "24"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The diagonals of a rhombus bisect each other at right angles, so they cut it into four congruent right triangles with legs 3 and 4 — half of each diagonal. The side is the hypotenuse: √(3² + 4²) = 5.",
                "7 is the average of 6 and 8, which has no geometric basis here. The side is a hypotenuse, so it comes from the Pythagorean Theorem rather than from averaging.",
                "10 uses the FULL diagonals as legs: √(6² + 8²) = 10. But each right triangle inside the rhombus has HALF of each diagonal for its legs, since the diagonals bisect each other.",
                "24 is the product 6 × 8, or twice the rhombus's area. Area = ½d₁d₂ = 24 here, so this is a real quantity in the problem — just not the side length."
              ],
              "hint": "The diagonals cut the rhombus into four right triangles. Be careful about how much of each diagonal forms a leg."
            }
          ]
        },
        {
          "id": "triangle-centers",
          "title": "Triangle Centers & the Euler Line",
          "blurb": "The four classical centers — centroid, incenter, circumcenter, orthocenter — what each is built from, where each lives, and the line three of them share.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "triangles-review",
            "properties-of-triangles"
          ],
          "questions": [
            {
              "id": "tct-001",
              "topic": "Triangle Centers",
              "difficulty": "easy",
              "stem": "Which triangle center is formed by the intersection of the three altitudes of a triangle?",
              "choices": [
                "Incenter",
                "Centroid",
                "Circumcenter",
                "Orthocenter"
              ],
              "correctIndex": 3,
              "explanations": [
                "The incenter is where the three ANGLE BISECTORS meet. Altitudes and angle bisectors coincide only in special triangles, so the two centers are generally different points.",
                "The centroid is where the three MEDIANS meet — the segments from each vertex to the midpoint of the opposite side. A median goes to a midpoint; an altitude goes in perpendicular. Those are different lines unless the triangle is isosceles about that vertex.",
                "The circumcenter is where the three PERPENDICULAR BISECTORS of the sides meet. Those are perpendicular to the sides like altitudes are, but they start at the midpoints rather than at the opposite vertices.",
                "Correct. An altitude runs from a vertex perpendicular to the opposite side, and all three are concurrent at the orthocenter. In an obtuse triangle two of the altitudes have to be extended outside the figure to meet, so the orthocenter itself lands outside the triangle."
              ],
              "hint": "Match the line to the center: altitudes, medians, angle bisectors, and perpendicular bisectors each produce a different point of concurrency.",
              "diagram": "<svg viewBox=\"0 0 240 140\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"35,120 205,120 140,25\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"140\" y1=\"25\" x2=\"140\" y2=\"120\" stroke=\"#1f2024\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><line x1=\"35\" y1=\"120\" x2=\"175\" y2=\"65\" stroke=\"#1f2024\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><line x1=\"205\" y1=\"120\" x2=\"96\" y2=\"62\" stroke=\"#1f2024\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><path d=\"M 130,120 L 130,110 L 140,110\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><circle cx=\"140\" cy=\"82\" r=\"3.2\" fill=\"#c0392b\"/><text x=\"146\" y=\"80\" fill=\"#c0392b\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">orthocenter</text></svg>",
              "diagramCaption": "The three altitudes are concurrent. Not drawn to scale"
            },
            {
              "id": "tct-002",
              "topic": "Triangle Centers",
              "difficulty": "medium",
              "stem": "Which triangle center is ALWAYS equidistant from the three vertices of a triangle?",
              "choices": [
                "Orthocenter",
                "Circumcenter",
                "Centroid",
                "Incenter"
              ],
              "correctIndex": 1,
              "explanations": [
                "The orthocenter is the concurrency of the altitudes. It has no equidistance property at all — in an obtuse triangle it is not even inside the figure.",
                "Correct. The circumcenter is the center of the circumscribed circle, the one passing through all three vertices. Every vertex sits on that circle, so every vertex is exactly one radius away. It is where the perpendicular bisectors meet, and a perpendicular bisector is precisely the set of points equidistant from two endpoints.",
                "The centroid is the balance point, dividing each median 2:1. It is equidistant from the vertices only when the triangle is equilateral.",
                "The incenter is equidistant from the three SIDES, not the vertices — it is the center of the inscribed circle, which touches each side rather than passing through each corner."
              ],
              "hint": "One of these four centers is the center of a circle. Ask which circle — the one through the corners, or the one tucked inside touching the edges?",
              "diagram": "<svg viewBox=\"0 0 240 140\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"35,120 205,120 140,25\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><circle cx=\"120\" cy=\"82\" r=\"88\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.5\"/><circle cx=\"120\" cy=\"82\" r=\"3.2\" fill=\"#c0392b\"/><line x1=\"120\" y1=\"82\" x2=\"35\" y2=\"120\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"82\" x2=\"205\" y2=\"120\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"82\" x2=\"140\" y2=\"25\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"126\" y=\"78\" fill=\"#c0392b\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">R</text></svg>",
              "diagramCaption": "The circumscribed circle passes through all three vertices. Not drawn to scale"
            },
            {
              "id": "tct-003",
              "topic": "Triangle Centers",
              "difficulty": "medium",
              "stem": "Which triangle center is ALWAYS equidistant from the three SIDES of a triangle?",
              "choices": [
                "Centroid",
                "Orthocenter",
                "Circumcenter",
                "Incenter"
              ],
              "correctIndex": 3,
              "explanations": [
                "The centroid is the balance point of the triangle, found from the medians. Its distances to the three sides are generally all different.",
                "The orthocenter is where the altitudes meet. Altitudes measure perpendicular distance from a VERTEX to a side, which is a different quantity from the distance from the center to each side — and the orthocenter has no equidistance property.",
                "The circumcenter is equidistant from the three VERTICES, not the sides. Swapping these two is the most common error on this question.",
                "Correct. The incenter is the center of the inscribed circle — the incircle — which is tangent to all three sides, so the perpendicular distance from the incenter to each side is one radius. It is where the angle bisectors meet, and an angle bisector is the set of points equidistant from the two sides forming the angle."
              ],
              "hint": "The inscribed circle touches each side once. What must be true of the distance from its center to each side?",
              "diagram": "<svg viewBox=\"0 0 240 140\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"35,120 205,120 140,25\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><circle cx=\"128\" cy=\"87\" r=\"30\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.5\"/><circle cx=\"128\" cy=\"87\" r=\"3.2\" fill=\"#c0392b\"/><line x1=\"128\" y1=\"87\" x2=\"128\" y2=\"120\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"134\" y=\"84\" fill=\"#c0392b\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">r</text></svg>",
              "diagramCaption": "The inscribed circle is tangent to all three sides. Not drawn to scale"
            },
            {
              "id": "tct-004",
              "topic": "Triangle Centers",
              "difficulty": "medium",
              "stem": "The centroid of a triangle divides each median into two segments. What is the ratio of the vertex-to-centroid segment to the centroid-to-midpoint segment?",
              "choices": [
                "3 : 2",
                "2 : 1",
                "1 : 1",
                "3 : 1"
              ],
              "correctIndex": 1,
              "explanations": [
                "3 : 2 does not describe either split. It may be a half-memory of the fact that the vertex-to-centroid piece is 2/3 of the whole median — but 2/3 to 1/3 reduces to 2 : 1, not 3 : 2.",
                "Correct. The centroid splits every median so that the piece connecting to the vertex is twice as long as the piece connecting to the side midpoint — a 2 : 1 ratio. Equivalently, the centroid sits 2/3 of the way along each median from its vertex.",
                "1 : 1 would mean the centroid is the midpoint of each median. It is not; that would make it the balance point of the three midpoints rather than of the triangle's area.",
                "3 : 1 would put the centroid 3/4 of the way from the vertex. The correct fraction is 2/3."
              ],
              "hint": "The centroid sits two-thirds of the way from each vertex. Turn that fraction into a ratio of the two pieces.",
              "diagram": "<svg viewBox=\"0 0 240 140\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"30,120 210,120 120,25\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"120\" y1=\"25\" x2=\"120\" y2=\"120\" stroke=\"#1f2024\" stroke-width=\"1.8\" stroke-linecap=\"round\"/><circle cx=\"120\" cy=\"87\" r=\"3.2\" fill=\"#c0392b\"/><circle cx=\"120\" cy=\"120\" r=\"3.2\" fill=\"#6b6b76\"/><text x=\"126\" y=\"60\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">2 parts</text><text x=\"126\" y=\"110\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">1 part</text><text x=\"60\" y=\"118\" fill=\"#6b6b76\" font-size=\"10\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">midpoint</text></svg>",
              "diagramCaption": "The centroid divides the median 2 : 1 from the vertex. Not drawn to scale"
            },
            {
              "id": "tct-005",
              "topic": "The Euler Line",
              "difficulty": "hard",
              "stem": "Leonhard Euler proved that three triangle centers always lie on a single line (the Euler Line). Which center is NOT on the Euler Line?",
              "choices": [
                "Circumcenter",
                "Orthocenter",
                "Incenter",
                "Centroid"
              ],
              "correctIndex": 2,
              "explanations": [
                "The circumcenter is one of the three that ARE collinear. It sits at one end of the Euler Line.",
                "The orthocenter is on the Euler Line, at the opposite end from the circumcenter.",
                "Correct. The incenter is the odd one out. The circumcenter, centroid and orthocenter are always collinear — and the centroid always lies between the other two, twice as far from the orthocenter as from the circumcenter. The incenter joins them only when the triangle is isosceles or equilateral, where symmetry forces every center onto the axis.",
                "The centroid is on the Euler Line, and in a specific place: exactly one third of the way from the circumcenter to the orthocenter."
              ],
              "hint": "Three of these four are built from perpendicularity or from midpoints. The fourth is built from angles — and that is the one that breaks ranks.",
              "diagram": "<svg viewBox=\"0 0 240 136\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"30,118 212,118 92,28\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"24\" y1=\"96\" x2=\"220\" y2=\"60\" stroke=\"#c0392b\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><circle cx=\"58\" cy=\"90\" r=\"3.2\" fill=\"#c0392b\"/><circle cx=\"112\" cy=\"80\" r=\"3.2\" fill=\"#c0392b\"/><circle cx=\"166\" cy=\"70\" r=\"3.2\" fill=\"#c0392b\"/><text x=\"30\" y=\"84\" fill=\"#c0392b\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">O</text><text x=\"108\" y=\"74\" fill=\"#c0392b\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">G</text><text x=\"170\" y=\"64\" fill=\"#c0392b\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">H</text><circle cx=\"96\" cy=\"96\" r=\"3.2\" fill=\"#1f2024\"/><text x=\"78\" y=\"108\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">I</text></svg>",
              "diagramCaption": "O, G and H are collinear; I sits off the line. Not drawn to scale"
            },
            {
              "id": "tct-006",
              "topic": "Triangle Centers",
              "difficulty": "easy",
              "stem": "Which set of lines intersects at the circumcenter of a triangle?",
              "choices": [
                "Altitudes from vertices to opposite sides",
                "Medians from vertices to midpoints",
                "Perpendicular bisectors of the sides",
                "Angle bisectors of the vertices"
              ],
              "correctIndex": 2,
              "explanations": [
                "Altitudes meet at the orthocenter. They are perpendicular to the sides, like perpendicular bisectors, but they pass through the opposite vertex rather than through the side's midpoint.",
                "Medians meet at the centroid. They pass through midpoints, like perpendicular bisectors, but they run to the opposite vertex rather than perpendicular to the side.",
                "Correct. The perpendicular bisectors of the three sides are concurrent at the circumcenter. Each one is the locus of points equidistant from the two endpoints of its side, so their common point is equidistant from all three vertices — which is exactly the center of the circumscribed circle.",
                "Angle bisectors meet at the incenter."
              ],
              "hint": "The perpendicular bisector of a segment is the set of points equidistant from its endpoints. What does intersecting three of them guarantee?"
            },
            {
              "id": "tct-007",
              "topic": "Triangle Centers",
              "difficulty": "hard",
              "stem": "In an obtuse triangle, where do the circumcenter and orthocenter lie?",
              "choices": [
                "Both inside the triangle",
                "Both outside the triangle",
                "The circumcenter inside, the orthocenter outside",
                "Both on the longest side"
              ],
              "correctIndex": 1,
              "explanations": [
                "Both are inside only when the triangle is acute. The obtuse case is exactly where that stops being true, which is why it gets asked.",
                "Correct. In an obtuse triangle both fall outside. The circumcenter moves across the longest side because the circumscribed circle's center must sit on the far side of it, and the orthocenter moves out because two of the three altitudes only meet the opposite sides once those sides are extended. The centroid, by contrast, is always inside — it is an average of the vertices.",
                "This mixes the two up, and neither half is right for an obtuse triangle. Both leave the figure together.",
                "Only in a right triangle does a center land on a side: there the circumcenter is the midpoint of the hypotenuse, and the orthocenter is the right-angle vertex."
              ],
              "hint": "Think about the right-triangle case first — the circumcenter sits on the hypotenuse. Now push the angle past 90° and ask which way it keeps moving."
            },
            {
              "id": "tct-008",
              "topic": "Midsegments",
              "difficulty": "medium",
              "stem": "A midsegment of a triangle joins the midpoints of two sides. If the third side measures 18 cm, what is the length of that midsegment, and how is it oriented?",
              "choices": [
                "9 cm, parallel to the third side",
                "9 cm, perpendicular to the third side",
                "36 cm, parallel to the third side",
                "18 cm, parallel to the third side"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The Triangle Midsegment Theorem gives both facts at once: the midsegment is parallel to the third side and exactly half its length. Half of 18 cm is 9 cm.",
                "The length is right but the orientation is not. Perpendicularity is not part of the theorem, and the midsegment is generally not perpendicular to anything.",
                "36 cm doubles instead of halving. The midsegment is the shorter of the two — it spans a smaller, similar triangle cut off at the top.",
                "18 cm would make the midsegment congruent to the third side. That happens in a parallelogram, not here: the midsegment and the third side are two parallel sides of a trapezoid whose lengths differ by a factor of two."
              ],
              "hint": "The midsegment cuts off a small triangle similar to the original, with a scale factor of one half. That factor answers both parts.",
              "diagram": "<svg viewBox=\"0 0 240 145\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"30,120 210,120 110,25\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"70\" y1=\"72\" x2=\"160\" y2=\"72\" stroke=\"#1f2024\" stroke-width=\"2.2\" stroke-linecap=\"round\"/><circle cx=\"70\" cy=\"72\" r=\"3.2\" fill=\"#1f2024\"/><circle cx=\"160\" cy=\"72\" r=\"3.2\" fill=\"#1f2024\"/><text x=\"98\" y=\"66\" fill=\"#1f2024\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">midsegment</text><text x=\"104\" y=\"136\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">18 cm</text></svg>",
              "diagramCaption": "The midsegment joins two side midpoints. Not drawn to scale"
            },
            {
              "id": "tct-009",
              "topic": "Right Triangle Altitudes",
              "difficulty": "hard",
              "stem": "An altitude drawn to the hypotenuse of a right triangle splits it into segments of length 4 and 9. What is the length of the altitude?",
              "choices": [
                "6",
                "6.5",
                "13",
                "36"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The altitude to the hypotenuse is the geometric mean of the two segments it creates: h = √(a·b) = √(4 × 9) = √36 = 6. This follows from the two smaller triangles being similar to each other, which gives h/4 = 9/h, so h² = 36.",
                "6.5 is the arithmetic mean, (4 + 9)/2. The relationship here is a GEOMETRIC mean — multiply and take the root, don't add and halve.",
                "13 is the whole hypotenuse, 4 + 9. That is the segment the altitude divides, not the altitude itself.",
                "36 is h², one step short. The equation h² = 4 × 9 still needs a square root taken."
              ],
              "hint": "The altitude creates two smaller triangles, each similar to the whole. Set up the proportion h/4 = 9/h and see what it gives.",
              "diagram": "<svg viewBox=\"0 0 240 136\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"25,110 215,110 77,25\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"77\" y1=\"25\" x2=\"77\" y2=\"110\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><path d=\"M 77,98 L 89,98 L 89,110\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><path d=\"M 68,34 L 78,40 L 72,48\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"50\" y=\"126\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4</text><text x=\"140\" y=\"126\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">9</text><text x=\"82\" y=\"72\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">h</text></svg>",
              "diagramCaption": "The altitude falls to the hypotenuse. Not drawn to scale"
            },
            {
              "id": "tct-010",
              "topic": "Triangle Inequality",
              "difficulty": "medium",
              "stem": "Two sides of a triangle measure 7 and 10. What is the full range of possible lengths for the third side?",
              "choices": [
                "Greater than 3 and less than 17",
                "Greater than 7 and less than 10",
                "Greater than 0 and less than 17",
                "Greater than 3 and less than 10"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The Triangle Inequality has to hold all three ways. The third side must be less than 7 + 10 = 17, and greater than 10 − 7 = 3 (otherwise the two given sides could not reach across it). So 3 < x < 17, with both bounds strict.",
                "7 to 10 confuses the range of the third side with the two given lengths themselves. Nothing requires the third side to fall between them — a triangle with sides 7, 10, 15 is perfectly valid.",
                "Getting the upper bound right but dropping the lower one misses half the constraint. A third side of 1 fails: 7 + 1 = 8, which is less than 10, so those three lengths cannot close.",
                "The lower bound is right but the upper is not. 10 is the length of a given side, not a cap — the sum 7 + 10 = 17 is what caps it."
              ],
              "hint": "Two conditions, not one: the third side must be short enough for the other two to reach across, and long enough that they cannot swallow it."
            },
            {
              "id": "tct-011",
              "topic": "Triangle Centers",
              "difficulty": "medium",
              "stem": "In which type of triangle do the centroid, incenter, circumcenter and orthocenter all coincide at a single point?",
              "choices": [
                "Any isosceles triangle",
                "Any right triangle",
                "An equilateral triangle",
                "Any obtuse triangle"
              ],
              "correctIndex": 2,
              "explanations": [
                "An isosceles triangle has one axis of symmetry, so all four centers land ON that axis — which is why the incenter joins the Euler Line there. But they are still four distinct points spread along it unless the third side matches too.",
                "In a right triangle the centers are conspicuously spread out: the circumcenter sits at the hypotenuse's midpoint and the orthocenter at the right-angle vertex, which are different points.",
                "Correct. An equilateral triangle has three axes of symmetry, and every center must lie on all of them. Three distinct lines meet in at most one point, so all four centers collapse together. Each cevian is simultaneously a median, an altitude, an angle bisector and a perpendicular bisector.",
                "Obtuse triangles push the circumcenter and orthocenter outside the figure entirely, while the centroid stays inside — about as far from coinciding as they get."
              ],
              "hint": "Each center must sit on every axis of symmetry the triangle has. How many axes force them all onto one point?"
            },
            {
              "id": "tct-012",
              "topic": "The Euler Line",
              "difficulty": "hard",
              "stem": "On the Euler Line, the centroid G lies between the circumcenter O and the orthocenter H. What is the ratio OG : GH?",
              "choices": [
                "1 : 1",
                "1 : 2",
                "2 : 1",
                "1 : 3"
              ],
              "correctIndex": 1,
              "explanations": [
                "1 : 1 would make the centroid the midpoint of OH. It is not — the centroid sits noticeably closer to the circumcenter.",
                "Correct. OG : GH = 1 : 2. The centroid is one third of the way from the circumcenter to the orthocenter, so OH = 3·OG. The same 2 : 1 proportion that governs the medians reappears here, which is not a coincidence: both come from the homothety centered at G with ratio −2 that carries the medial triangle onto the original.",
                "2 : 1 reverses it, putting the centroid closer to the orthocenter. The distances are the other way round.",
                "1 : 3 would make OH = 4·OG. The correct factor is 3."
              ],
              "hint": "The centroid is one third of the way along OH. Turn that fraction into the ratio of the two pieces."
            },
            {
              "id": "tct-013",
              "topic": "Centroid Ratio",
              "difficulty": "medium",
              "stem": "The centroid of a triangle divides a median so that the longer piece measures 10. What is the length of the entire median?",
              "choices": [
                "12",
                "15",
                "20",
                "30"
              ],
              "correctIndex": 1,
              "explanations": [
                "12 would follow from a 5:1 split. The centroid splits each median in a 2:1 ratio measured from the vertex, so the two pieces are 10 and 5.",
                "Correct. The centroid divides every median in a 2:1 ratio, with the longer piece adjacent to the vertex. If the longer piece is 10, the shorter is 5, and the full median is 10 + 5 = 15.",
                "20 doubles the longer piece, which would be right only if the centroid were the MIDPOINT of the median. It is not — the midpoint of a median is not a triangle center at all.",
                "30 triples the longer piece. The whole median is 3 halves of the shorter piece, not 3 times the longer one — the ratio applies to the parts, not to the total directly."
              ],
              "hint": "The 2:1 ratio describes the two pieces relative to each other. Find the shorter piece first, then add."
            },
            {
              "id": "tct-014",
              "topic": "Incenter",
              "difficulty": "easy",
              "stem": "Which triangle center is always equidistant from all three SIDES of the triangle?",
              "choices": [
                "Centroid",
                "Circumcenter",
                "Incenter",
                "Orthocenter"
              ],
              "correctIndex": 2,
              "explanations": [
                "The centroid is the balance point, where the medians meet. It is the center of mass, but its distances to the three sides are generally all different.",
                "The circumcenter is equidistant from the three VERTICES, not the sides — that shared distance is the circumradius. Vertices and sides are the distinction the question turns on.",
                "Correct. The incenter is where the three angle bisectors meet, and a point on an angle bisector is equidistant from the two sides forming that angle. Meeting all three bisectors makes it equidistant from all three sides, which is exactly the radius of the inscribed circle.",
                "The orthocenter is where the altitudes meet. It carries no equidistance property at all, and in an obtuse triangle it falls outside the triangle entirely."
              ],
              "hint": "Two of these centers are equidistant from something. Sort out which one measures to sides and which to vertices."
            },
            {
              "id": "tct-015",
              "topic": "Circumcenter Location",
              "difficulty": "hard",
              "stem": "In which type of triangle does the circumcenter lie OUTSIDE the triangle?",
              "choices": [
                "Acute",
                "Right",
                "Obtuse",
                "Equilateral"
              ],
              "correctIndex": 2,
              "explanations": [
                "In an acute triangle the circumcenter lies strictly inside. All three angles being under 90° keeps the perpendicular bisectors meeting within the figure.",
                "In a right triangle the circumcenter lies exactly ON the triangle — at the midpoint of the hypotenuse. That is a boundary case rather than an outside one, and it is why the hypotenuse is a diameter of the circumscribed circle.",
                "Correct. In an obtuse triangle the circumcenter falls outside, on the far side of the longest side from the obtuse angle. The circumscribed circle still passes through all three vertices; its center simply is not enclosed by the triangle.",
                "An equilateral triangle is a special acute triangle in which the circumcenter, incenter, centroid and orthocenter all coincide at a single interior point."
              ],
              "hint": "Work through the three angle classifications in order, and remember the right-triangle case lands somewhere very specific."
            },
            {
              "id": "tct-016",
              "topic": "The Euler Line",
              "difficulty": "hard",
              "stem": "Which three triangle centers are always collinear, lying on the Euler line?",
              "choices": [
                "Incenter, centroid, circumcenter",
                "Centroid, circumcenter, orthocenter",
                "Incenter, orthocenter, circumcenter",
                "All four classical centers"
              ],
              "correctIndex": 1,
              "explanations": [
                "The incenter is the odd one out — it does NOT lie on the Euler line in a general triangle. Swapping it in for the orthocenter is the usual way this question is missed.",
                "Correct. The centroid, circumcenter and orthocenter are always collinear, and the line through them is the Euler line. The centroid sits between the other two and divides the segment in a 2:1 ratio, with the longer part toward the orthocenter.",
                "This again includes the incenter, which is not on the Euler line except in special triangles such as isosceles ones, where symmetry forces every center onto the axis.",
                "All four coincide only in an equilateral triangle. In that case the Euler line degenerates to a single point, so it is a special case rather than the general statement."
              ],
              "hint": "One of the four classical centers is famously absent from this line. Identify which, and the rest follows."
            },
            {
              "id": "tct-017",
              "topic": "Orthocenter",
              "difficulty": "medium",
              "stem": "A triangle has a 120° angle. Where is its orthocenter located?",
              "choices": [
                "Inside the triangle",
                "On the triangle, at a vertex",
                "Outside the triangle",
                "At the midpoint of the longest side"
              ],
              "correctIndex": 2,
              "explanations": [
                "The orthocenter lies inside only when all three angles are acute. A 120° angle makes this triangle obtuse, so the interior case does not apply.",
                "The orthocenter sits at a vertex only in a RIGHT triangle, where it lands on the right-angle vertex because two of the altitudes are the legs themselves. A 120° angle is obtuse, not right.",
                "Correct. In an obtuse triangle two of the altitudes must be extended beyond the triangle before they meet, so their point of concurrency falls outside the figure. The 120° angle guarantees this.",
                "The midpoint of the longest side is where the CIRCUMCENTER lands in a right triangle. It is not a location the orthocenter takes, and this triangle is obtuse in any case."
              ],
              "hint": "Classify the triangle by its largest angle first, then recall how the altitudes behave in that case."
            },
            {
              "id": "tct-018",
              "topic": "Reasoning: Choose the Centre",
              "difficulty": "hard",
              "stem": "A town wants to place a water tower equally distant from three straight roads that form a triangle. Which centre locates it, and why?",
              "choices": [
                "The circumcentre, because it is equidistant from the three vertices.",
                "The incentre, because it is equidistant from the three SIDES.",
                "The centroid, because it balances the triangle.",
                "The orthocentre, because the altitudes measure distance to the sides."
              ],
              "correctIndex": 1,
              "explanations": [
                "The circumcentre is equidistant from the three VERTICES — the right choice for reaching three towns, not three roads.",
                "Correct. The incentre is where the angle bisectors meet, and it is equidistant from the three sides. Roads are the sides, so this is the point that is equally far from each road.",
                "The centroid is the balance point, the intersection of the medians. It is generally not equidistant from anything in particular.",
                "The orthocentre is where the altitudes meet. Altitudes are perpendicular to the sides, but their intersection is not equidistant from them."
              ],
              "hint": "Distance to a LINE is measured perpendicular. Which centre is defined by equal perpendicular distances?",
              "kind": "reasoning"
            },
            {
              "id": "tct-019",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "A student claims every triangle centre always lies inside the triangle. Which centre disproves this, and when?",
              "choices": [
                "The centroid, when the triangle is scalene.",
                "The incentre, when the triangle is obtuse.",
                "The circumcentre, which falls outside whenever the triangle is obtuse.",
                "None — all four centres are always interior."
              ],
              "correctIndex": 2,
              "explanations": [
                "The centroid is always interior: it is the average of the three vertices, so it can never leave the triangle.",
                "The incentre is always interior too — it is the centre of a circle that fits inside the triangle.",
                "Correct. The circumcentre lies outside an obtuse triangle (and exactly on the hypotenuse's midpoint for a right triangle). The orthocentre also leaves an obtuse triangle.",
                "Two of them can be exterior. Only the centroid and incentre are always inside."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "polygon-area-quads",
          "title": "Polygon Angles, Area & Quadrilaterals",
          "blurb": "Interior and exterior angle sums, the quadrilateral property chain, and the area formulas for rhombus, parallelogram, trapezoid and regular polygon — each tied to the dissection it comes from.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "polygons",
            "quadrilaterals"
          ],
          "questions": [
            {
              "id": "pqa-001",
              "topic": "Regular Polygons",
              "difficulty": "medium",
              "stem": "What is the measure of ONE interior angle of a regular octagon (n = 8)?",
              "choices": [
                "45°",
                "135°",
                "140°",
                "1080°"
              ],
              "correctIndex": 1,
              "explanations": [
                "45° is one EXTERIOR angle, 360°/8. The interior angle is its supplement.",
                "Correct. Two routes give the same answer. Via the exterior angle: each is 360°/8 = 45°, so each interior angle is 180° − 45° = 135°. Or directly: (n − 2)·180°/n = (6)(180°)/8 = 1080°/8 = 135°.",
                "140° is the interior angle of a regular NONAGON (n = 9). For an octagon the value is 135°.",
                "1080° is the SUM of all eight interior angles, (8 − 2)(180°). The question asks for one of them, so divide by 8."
              ],
              "hint": "Go through the exterior angle — they always total 360°, whatever the polygon. Then take the supplement.",
              "diagram": "<svg viewBox=\"0 0 240 168\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"95,18 145,18 180,53 180,103 145,138 95,138 60,103 60,53\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><text x=\"146\" y=\"44\" fill=\"#1f2024\" font-size=\"14\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><text x=\"78\" y=\"158\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">regular octagon</text></svg>",
              "diagramCaption": "All eight interior angles are equal. Not drawn to scale"
            },
            {
              "id": "pqa-002",
              "topic": "Quadrilateral Properties",
              "difficulty": "medium",
              "stem": "Which quadrilateral has diagonals that are perpendicular and bisect each other, but are NOT necessarily congruent?",
              "choices": [
                "Parallelogram",
                "Rectangle",
                "Rhombus",
                "Isosceles trapezoid"
              ],
              "correctIndex": 2,
              "explanations": [
                "A general parallelogram has diagonals that bisect each other, but they are not perpendicular unless it is also a rhombus. Perpendicularity is the extra condition.",
                "A rectangle has diagonals that bisect each other AND are congruent — but they are not perpendicular unless it is a square. This is the mirror image of the rhombus.",
                "Correct. A rhombus has four equal sides, and its diagonals are perpendicular bisectors of each other. They are generally different lengths: d₁ ≠ d₂ unless the rhombus is a square. That is exactly what the question's \"not necessarily congruent\" is testing.",
                "An isosceles trapezoid has congruent diagonals, but they do not bisect each other at all — only one pair of its sides is parallel."
              ],
              "hint": "Sort the four by what their diagonals do: bisect, be congruent, be perpendicular. You want the one with bisect and perpendicular but not congruent.",
              "diagram": "<svg viewBox=\"0 0 240 156\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"120,18 200,72 120,126 40,72\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"120\" y1=\"18\" x2=\"120\" y2=\"126\" stroke=\"#1f2024\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"72\" x2=\"200\" y2=\"72\" stroke=\"#1f2024\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><path d=\"M 120,60 L 132,60 L 132,72\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"126\" y=\"46\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">d₁</text><text x=\"160\" y=\"66\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">d₂</text><text x=\"64\" y=\"146\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">d₁ ≠ d₂ in general</text></svg>",
              "diagramCaption": "A rhombus: diagonals perpendicular, bisecting, unequal. Not drawn to scale"
            },
            {
              "id": "pqa-003",
              "topic": "Area of Quadrilaterals",
              "difficulty": "medium",
              "stem": "A rhombus has diagonals of length 10 cm and 24 cm. What is its area?",
              "choices": [
                "240 cm²",
                "120 cm²",
                "60 cm²",
                "34 cm²"
              ],
              "correctIndex": 1,
              "explanations": [
                "240 cm² multiplies the diagonals without halving. That product is the area of the RECTANGLE the rhombus is inscribed in — exactly twice too big.",
                "Correct. A rhombus has area A = ½d₁d₂ = ½(10)(24) = 120 cm². The reason: the perpendicular diagonals cut it into four right triangles, each with legs d₁/2 and d₂/2 and area ½(d₁/2)(d₂/2) = d₁d₂/8. Four of those give 4(d₁d₂/8) = ½d₁d₂.",
                "60 cm² quarters the product instead of halving it — the area of two of the four right triangles rather than all four.",
                "34 cm² adds the diagonals. Adding lengths gives a length, not an area; the units alone rule this out."
              ],
              "hint": "The diagonals cut the rhombus into four congruent right triangles. Find one triangle's area and multiply.",
              "diagram": "<svg viewBox=\"0 0 240 142\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"120,16 208,72 120,128 32,72\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"120\" y1=\"16\" x2=\"120\" y2=\"128\" stroke=\"#1f2024\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><line x1=\"32\" y1=\"72\" x2=\"208\" y2=\"72\" stroke=\"#1f2024\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><path d=\"M 120,60 L 132,60 L 132,72\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"126\" y=\"44\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">10</text><text x=\"160\" y=\"66\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">24</text></svg>",
              "diagramCaption": "Diagonals 10 cm and 24 cm. Not drawn to scale"
            },
            {
              "id": "pqa-004",
              "topic": "Area of Quadrilaterals",
              "difficulty": "easy",
              "stem": "A parallelogram has a base of 14 cm and a slant side of 9 cm. Its perpendicular height is 6 cm. What is its area?",
              "choices": [
                "126 cm²",
                "84 cm²",
                "54 cm²",
                "46 cm²"
              ],
              "correctIndex": 1,
              "explanations": [
                "126 cm² uses the slant side (14 × 9) instead of the height. The slant side is longer than the perpendicular height, so this always overestimates.",
                "Correct. A parallelogram's area is A = bh = 14 × 6 = 84 cm², using the PERPENDICULAR height. Cutting the right triangle off one end and translating it to the other turns the parallelogram into a 14 × 6 rectangle of exactly the same area — which is where the formula comes from.",
                "54 cm² is 9 × 6, pairing the slant side with the height. The base and the height must be perpendicular to each other, and the base here is 14.",
                "46 cm² is the perimeter, 2(14 + 9). Perimeter is a length; area needs a product of two perpendicular lengths."
              ],
              "hint": "Slide the triangle from one end of the parallelogram to the other. What shape do you get, and which two lengths are its sides?",
              "diagram": "<svg viewBox=\"0 0 240 138\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"40,110 200,110 215,40 55,40\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"55\" y1=\"40\" x2=\"55\" y2=\"110\" stroke=\"#1f2024\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><path d=\"M 55,98 L 67,98 L 67,110\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"60\" y=\"78\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">6</text><text x=\"110\" y=\"128\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">14</text><text x=\"28\" y=\"76\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">9</text></svg>",
              "diagramCaption": "Height is measured perpendicular to the base. Not drawn to scale"
            },
            {
              "id": "pqa-005",
              "topic": "Area of Quadrilaterals",
              "difficulty": "hard",
              "stem": "A parallelogram has sides of length a and b with an included angle y. Which expression gives its area?",
              "choices": [
                "ab",
                "½ab·sin(y)",
                "ab·sin(y)",
                "ab·cos(y)"
              ],
              "correctIndex": 2,
              "explanations": [
                "ab is the area only when y = 90°, making the parallelogram a rectangle. For any other angle the figure leans over and its area is strictly less.",
                "½ab·sin(y) is the area of the TRIANGLE formed by the two sides and the included angle. A parallelogram is two such triangles, so this is exactly half the answer.",
                "Correct. The height on base b is h = a·sin(y), so A = bh = ab·sin(y). This also handles the obtuse case correctly without a separate rule, because sin(180° − y) = sin(y) — the height is the same whether you measure the angle or its supplement.",
                "ab·cos(y) uses the wrong projection. Cosine gives the component ALONG the base, which is the horizontal offset, not the perpendicular height. It would also make the area zero at y = 90°, when it should be greatest."
              ],
              "hint": "Drop the height from one vertex. Which trig ratio turns the slant side a and the angle y into that perpendicular height?"
            },
            {
              "id": "pqa-006",
              "topic": "Polygon Angle Sums",
              "difficulty": "easy",
              "stem": "What is the sum of the interior angles of a convex polygon with 15 sides?",
              "choices": [
                "2340°",
                "2700°",
                "360°",
                "156°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The interior angle sum is (n − 2)·180°. For n = 15 that is 13 × 180° = 2340°. The −2 is there because a polygon with n sides splits into n − 2 triangles when you draw all the diagonals from a single vertex.",
                "2700° is 15 × 180°, skipping the −2. Drawing diagonals from one vertex of a 15-gon produces 13 triangles, not 15.",
                "360° is the sum of the EXTERIOR angles, which is 360° for every convex polygon regardless of n. The question asks about interior angles.",
                "156° is one interior angle of a REGULAR 15-gon, 2340°/15. The question asks for the total, and it does not say the polygon is regular."
              ],
              "hint": "Fan the polygon into triangles from one vertex. How many triangles does a 15-gon give, and what does each contribute?"
            },
            {
              "id": "pqa-007",
              "topic": "Polygon Angle Sums",
              "difficulty": "medium",
              "stem": "Each exterior angle of a regular polygon measures 24°. How many sides does the polygon have?",
              "choices": [
                "15",
                "12",
                "24",
                "156"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The exterior angles of any convex polygon total 360°, and in a regular polygon they are all equal, so n = 360°/24° = 15.",
                "12 would correspond to exterior angles of 30°. Check the division: 360/24 = 15, not 12.",
                "24 repeats the given angle measure. The number of sides is what 360° divides into, not the angle itself.",
                "156 is the INTERIOR angle, 180° − 24°. That is a real quantity here, but it is a number of degrees, not a count of sides."
              ],
              "hint": "Walk once around the polygon and you turn through a full revolution. Split that turn among the equal corners."
            },
            {
              "id": "pqa-008",
              "topic": "Quadrilateral Properties",
              "difficulty": "medium",
              "stem": "A quadrilateral has two distinct pairs of adjacent congruent sides and perpendicular diagonals, but only one diagonal is bisected. What is it?",
              "choices": [
                "Rhombus",
                "Kite",
                "Isosceles trapezoid",
                "Rectangle"
              ],
              "correctIndex": 1,
              "explanations": [
                "A rhombus has all FOUR sides congruent, and BOTH of its diagonals are bisected. The question specifies two distinct pairs and only one bisected diagonal, which rules it out.",
                "Correct. A kite is defined by two distinct pairs of adjacent congruent sides. Its diagonals are perpendicular, but only the diagonal joining the two vertices between unequal sides — the axis of symmetry — bisects the other. A rhombus is the special case where both pairs happen to be equal to each other.",
                "An isosceles trapezoid has one pair of PARALLEL sides and congruent legs, and its diagonals are congruent rather than perpendicular.",
                "A rectangle has congruent diagonals that bisect each other, and they are perpendicular only when it is a square. Its congruent sides come in OPPOSITE pairs, not adjacent ones."
              ],
              "hint": "\"Adjacent congruent sides\" is the giveaway — parallelograms pair up opposite sides instead.",
              "diagram": "<svg viewBox=\"0 0 240 156\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"120,14 190,80 120,142 50,80\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"120\" y1=\"14\" x2=\"120\" y2=\"142\" stroke=\"#1f2024\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><line x1=\"50\" y1=\"80\" x2=\"190\" y2=\"80\" stroke=\"#1f2024\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><path d=\"M 120,68 L 132,68 L 132,80\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"146\" y=\"40\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">a</text><text x=\"84\" y=\"40\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">a</text><text x=\"154\" y=\"118\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">b</text><text x=\"76\" y=\"118\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">b</text></svg>",
              "diagramCaption": "Two distinct pairs of adjacent equal sides. Not drawn to scale"
            },
            {
              "id": "pqa-009",
              "topic": "Quadrilateral Properties",
              "difficulty": "hard",
              "stem": "A parallelogram is known to have congruent diagonals. What can you conclude?",
              "choices": [
                "It is a rhombus.",
                "It is a rectangle.",
                "It is a square.",
                "Nothing beyond it being a parallelogram."
              ],
              "correctIndex": 1,
              "explanations": [
                "Congruent diagonals do not give a rhombus — a rhombus is what you get from PERPENDICULAR diagonals. The two conditions produce the two different specializations of a parallelogram.",
                "Correct. A parallelogram with congruent diagonals is a rectangle. The diagonals of a parallelogram already bisect each other; making them congruent as well forces all four half-diagonals to be equal, which puts the four vertices on a common circle and makes every angle a right angle.",
                "A square needs both extra conditions at once: congruent diagonals AND perpendicular ones. Only one is given here, so the figure may be a non-square rectangle.",
                "There is definitely more to conclude. Congruent diagonals are one of the standard sufficient conditions for a rectangle, so the extra information is not idle."
              ],
              "hint": "Follow the property chain: parallelogram plus perpendicular diagonals gives one figure, parallelogram plus congruent diagonals gives the other."
            },
            {
              "id": "pqa-010",
              "topic": "Area of Quadrilaterals",
              "difficulty": "medium",
              "stem": "A trapezoid has parallel sides of 8 cm and 14 cm and a height of 5 cm. What is its area?",
              "choices": [
                "110 cm²",
                "55 cm²",
                "27 cm²",
                "280 cm²"
              ],
              "correctIndex": 1,
              "explanations": [
                "110 cm² is (b₁ + b₂)h without halving, 22 × 5. The formula averages the two parallel sides before multiplying by the height.",
                "Correct. A trapezoid's area is A = ½(b₁ + b₂)h = ½(8 + 14)(5) = ½(22)(5) = 55 cm². The average of the two parallel sides, 11 cm, is the width of a rectangle with the same area and height.",
                "27 cm² adds all three numbers, 8 + 14 + 5. That mixes two parallel sides with a perpendicular height and produces neither an area nor a perimeter.",
                "280 cm² multiplies all three, 8 × 14 × 5. Three lengths multiplied give a volume, not an area — the units give it away."
              ],
              "hint": "Replace the trapezoid with a rectangle of the same height whose width is the average of the two parallel sides.",
              "diagram": "<svg viewBox=\"0 0 240 138\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"30,110 210,110 170,42 76,42\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"76\" y1=\"42\" x2=\"76\" y2=\"110\" stroke=\"#1f2024\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><path d=\"M 76,98 L 88,98 L 88,110\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"82\" y=\"80\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5</text><text x=\"112\" y=\"36\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">8</text><text x=\"112\" y=\"128\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">14</text></svg>",
              "diagramCaption": "The two parallel sides measure 8 cm and 14 cm. Not drawn to scale"
            },
            {
              "id": "pqa-011",
              "topic": "Regular Polygons",
              "difficulty": "hard",
              "stem": "A regular polygon with perimeter 60 cm has an apothem of 8 cm. What is its area?",
              "choices": [
                "480 cm²",
                "240 cm²",
                "68 cm²",
                "120 cm²"
              ],
              "correctIndex": 1,
              "explanations": [
                "480 cm² is the full product, apothem × perimeter. That double-counts: it is the area of all the triangles taken with the full side length as base rather than accounting for the ½ in each triangle's area.",
                "Correct. A regular polygon's area is A = ½(apothem)(perimeter) = ½(8)(60) = 240 cm². Splitting the polygon into triangles from the centre, each has base equal to a side and height equal to the apothem, so the total is ½ × apothem × (sum of all sides).",
                "68 cm² adds the apothem and perimeter. Adding two lengths cannot give an area.",
                "120 cm² is half the perimeter alone, or a slip that drops the apothem. Both quantities are needed."
              ],
              "hint": "Cut the polygon into triangles from the centre. Each has the apothem as its height — now add up all their bases at once."
            },
            {
              "id": "pqa-012",
              "topic": "Polygon Angle Sums",
              "difficulty": "medium",
              "stem": "A convex polygon has interior angles summing to 1440°. If it is regular, what is the measure of each exterior angle?",
              "choices": [
                "36°",
                "144°",
                "45°",
                "40°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. From (n − 2)180° = 1440° we get n − 2 = 8, so n = 10 — a decagon. Its exterior angles total 360° and are all equal, so each is 360°/10 = 36°.",
                "144° is each INTERIOR angle, 1440°/10. The exterior angle is its supplement, 180° − 144° = 36°.",
                "45° would correspond to an octagon (n = 8). Solving (n − 2)180 = 1440 gives n = 10, not 8 — the −2 is easy to drop here.",
                "40° would correspond to a nonagon (n = 9). Recheck the division: 1440/180 = 8, so n − 2 = 8 and n = 10."
              ],
              "hint": "Recover n from the interior sum first, then use the fact that exterior angles always total 360° whatever n turns out to be."
            },
            {
              "id": "pqa-013",
              "topic": "Interior Angle Sum",
              "difficulty": "easy",
              "stem": "Find the sum of the interior angles of a decagon (n = 10).",
              "choices": [
                "1260°",
                "1440°",
                "1800°",
                "360°"
              ],
              "correctIndex": 1,
              "explanations": [
                "1260° is the interior sum for a NONAGON (n = 9): (9 − 2)·180. Off-by-one on n is the most frequent slip in this formula.",
                "Correct. The interior angle sum of an n-gon is (n − 2)·180°. For n = 10 that is (10 − 2)·180 = 8 × 180 = 1440°. The (n − 2) counts the triangles the polygon splits into from one vertex.",
                "1800° is 10 × 180, which forgets to subtract 2 from n. That would be the sum if the polygon split into ten triangles rather than eight.",
                "360° is the sum of the EXTERIOR angles, which is 360° for every convex polygon regardless of n. Interior and exterior sums behave very differently: one grows with n, the other never changes."
              ],
              "hint": "Picture drawing every diagonal from a single vertex. How many triangles does an n-gon break into?"
            },
            {
              "id": "pqa-014",
              "topic": "Regular Polygons",
              "difficulty": "medium",
              "stem": "Each interior angle of a regular polygon measures 150°. How many sides does it have?",
              "choices": [
                "10",
                "12",
                "15",
                "24"
              ],
              "correctIndex": 1,
              "explanations": [
                "10 sides would give an exterior angle of 36° and an interior angle of 144°, not 150°. Close, but the arithmetic does not land.",
                "Correct. The fastest route goes through the exterior angle: interior 150° means exterior 180 − 150 = 30°. Since exterior angles of a regular n-gon each equal 360/n, we get n = 360/30 = 12.",
                "15 sides gives an exterior angle of 24° and an interior angle of 156°. This comes from dividing 360 by the interior angle's complement rather than its supplement.",
                "24 sides gives an exterior angle of 15° — the digits of 150 misread. The exterior angle here is 30°, not 15°."
              ],
              "hint": "Convert the interior angle to its exterior partner first. The exterior route avoids solving a fraction equation."
            },
            {
              "id": "pqa-015",
              "topic": "Exterior Angles",
              "difficulty": "easy",
              "stem": "One exterior angle of a regular polygon measures 24°. How many sides does the polygon have?",
              "choices": [
                "12",
                "15",
                "18",
                "24"
              ],
              "correctIndex": 1,
              "explanations": [
                "12 sides gives an exterior angle of 30°, not 24°. This comes from dividing 360 by 30 rather than by the given 24.",
                "Correct. The exterior angles of any convex polygon sum to 360°, and in a regular polygon they are all equal. So n = 360/24 = 15.",
                "18 sides gives an exterior angle of 20°. The division was performed with the wrong divisor.",
                "24 repeats the given angle as though it were the side count. The angle and the number of sides are related by division into 360, not by equality."
              ],
              "hint": "The exterior angles always total the same amount, no matter how many sides. Divide by the size of one."
            },
            {
              "id": "pqa-016",
              "topic": "Parallelogram Angles",
              "difficulty": "medium",
              "stem": "Consecutive angles of a parallelogram measure (2x + 10)° and (3x − 40)°. Find x.",
              "choices": [
                "42",
                "50",
                "30",
                "38"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Consecutive angles of a parallelogram are SUPPLEMENTARY. So (2x + 10) + (3x − 40) = 180, giving 5x − 30 = 180, then 5x = 210 and x = 42. The angles are 94° and 86°, which do sum to 180°.",
                "50 comes from setting the two expressions equal: 2x + 10 = 3x − 40 gives x = 50. That would be right for OPPOSITE angles, which are equal — but consecutive angles are supplementary.",
                "30 comes from setting the sum to 90° instead of 180°. Consecutive angles of a parallelogram straddle a pair of parallel sides cut by a transversal, so they are co-interior and sum to a straight angle.",
                "38 comes from solving 5x = 190, which mishandles the −30 constant. Moving −30 to the right side adds 30 to 180, giving 210."
              ],
              "hint": "Decide first whether consecutive angles in a parallelogram are equal or supplementary. The two lead to very different equations."
            },
            {
              "id": "pqa-017",
              "topic": "Rhombus Diagonals",
              "difficulty": "hard",
              "stem": "A rhombus has a side of length 10 and one diagonal of length 12. Find the other diagonal.",
              "choices": [
                "8",
                "16",
                "20",
                "6"
              ],
              "correctIndex": 1,
              "explanations": [
                "8 is HALF the other diagonal, not the whole thing. The right-triangle computation gives the half-diagonal; it still has to be doubled because the diagonals bisect each other.",
                "Correct. The diagonals bisect each other perpendicularly, so half of the given diagonal is 6, and the side of 10 is the hypotenuse of a right triangle with legs 6 and h. Then h = √(100 − 36) = 8, and the full second diagonal is 2 × 8 = 16.",
                "20 doubles the side length rather than the computed half-diagonal. The side is a hypotenuse in this configuration, not a piece of a diagonal.",
                "6 is half of the GIVEN diagonal, which was an intermediate value in the calculation rather than the answer."
              ],
              "hint": "Half of each diagonal forms the legs of a right triangle whose hypotenuse is the side. Solve for the leg, then remember to double."
            },
            {
              "id": "pqa-018",
              "topic": "Reasoning: Classification",
              "difficulty": "medium",
              "stem": "A quadrilateral has diagonals that bisect each other. What is the strongest conclusion, and why is nothing stronger justified?",
              "choices": [
                "It is a rectangle, because bisecting diagonals force right angles.",
                "It is a parallelogram — bisecting diagonals guarantee that and no more.",
                "It is a rhombus, because bisecting diagonals are perpendicular.",
                "It is a square, since only squares have bisecting diagonals."
              ],
              "correctIndex": 1,
              "explanations": [
                "Rectangles need CONGRUENT diagonals, which is a separate condition. Bisecting alone does not give right angles.",
                "Correct. Diagonals bisecting each other is exactly the condition for a parallelogram. Rectangles add congruent diagonals, rhombuses add perpendicular ones, and squares add both.",
                "Rhombuses need PERPENDICULAR diagonals. Bisecting and perpendicular are different properties.",
                "Every parallelogram has bisecting diagonals, and most are not squares."
              ],
              "hint": "Each extra property on the quadrilateral family tree buys exactly one extra guarantee.",
              "kind": "reasoning"
            },
            {
              "id": "pqa-019",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "hard",
              "stem": "A student computes the sum of the interior angles of a regular decagon as 10 × 180° = 1800°, then says each interior angle is 180°. What went wrong?",
              "choices": [
                "The interior angle sum is (n − 2)180° = 1440°, so each angle is 144°.",
                "The sum is right, but they should have divided by 20.",
                "The interior angle sum of any polygon is 360°.",
                "Nothing is wrong; a decagon's angles are each 180°."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The sum is (n − 2)180° = 8 × 180° = 1440°, and dividing by 10 gives 144° per angle.",
                "Dividing by 20 would give 90°, which is not right either — the error is in the sum, not the division.",
                "360° is the sum of the EXTERIOR angles, which is the same for every polygon. Interior angle sums grow with n.",
                "An interior angle of 180° would be a straight line, which cannot be a corner of a polygon — the answer gives itself away."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "circle-theorems-advanced",
          "title": "Circle Theorems (Advanced)",
          "blurb": "Inscribed angles, chords crossing inside and secants meeting outside, the tangent-secant power theorem, cyclic quadrilaterals, and sector area.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "circles"
          ],
          "questions": [
            {
              "id": "cta-001",
              "topic": "Inscribed Angles",
              "difficulty": "easy",
              "stem": "An inscribed angle intercepts an arc measuring 110°. What is the measure of the inscribed angle?",
              "choices": [
                "220°",
                "55°",
                "70°",
                "110°"
              ],
              "correctIndex": 1,
              "explanations": [
                "220° doubles the arc instead of halving it. That relationship runs the other way: the ARC is twice the inscribed angle, so 110° would be the angle and 220° the arc.",
                "Correct. An inscribed angle is always half the measure of its intercepted arc: ½(110°) = 55°.",
                "70° does not follow from any relationship here. It might come from 180° − 110°, which is the supplement — a fact about cyclic quadrilaterals, not about a single inscribed angle.",
                "110° repeats the arc. That would be the measure of the CENTRAL angle standing on the same arc; an inscribed angle on the same arc is half of it."
              ],
              "hint": "Central angle equals its arc. An inscribed angle on the same arc is half that.",
              "diagram": "<svg viewBox=\"0 0 240 148\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><circle cx=\"120\" cy=\"70\" r=\"55\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"120\" y1=\"15\" x2=\"78\" y2=\"105\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"15\" x2=\"165\" y2=\"100\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><circle cx=\"120\" cy=\"15\" r=\"3.2\" fill=\"#1f2024\"/><circle cx=\"78\" cy=\"105\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"165\" cy=\"100\" r=\"3.2\" fill=\"#7c3aed\"/><text x=\"112\" y=\"34\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><text x=\"94\" y=\"138\" fill=\"#7c3aed\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">arc = 110°</text></svg>",
              "diagramCaption": "The inscribed angle stands on the marked arc. Not drawn to scale"
            },
            {
              "id": "cta-002",
              "topic": "Intersecting Chords",
              "difficulty": "medium",
              "stem": "Two chords intersect inside a circle. The intercepted arcs are 80° and 40°. What is the measure of the vertical angles formed at their intersection?",
              "choices": [
                "20°",
                "120°",
                "40°",
                "60°"
              ],
              "correctIndex": 3,
              "explanations": [
                "20° halves the DIFFERENCE of the arcs, ½(80 − 40). The difference rule applies when the vertex is OUTSIDE the circle. Inside, the arcs add.",
                "120° is the sum of the two arcs without halving. The angle is half that sum, not the sum itself.",
                "40° repeats one of the given arcs. Both arcs contribute to the angle.",
                "Correct. When two chords intersect inside a circle, the angle is half the SUM of the two intercepted arcs: ½(80° + 40°) = ½(120°) = 60°."
              ],
              "hint": "Vertex inside the circle or outside? That decides whether the two arcs are added or subtracted before halving.",
              "diagram": "<svg viewBox=\"0 0 240 145\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><circle cx=\"120\" cy=\"72\" r=\"58\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"70\" y1=\"42\" x2=\"178\" y2=\"96\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"74\" y1=\"105\" x2=\"172\" y2=\"40\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><circle cx=\"122\" cy=\"72\" r=\"3.2\" fill=\"#c0392b\"/><text x=\"108\" y=\"24\" fill=\"#7c3aed\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">80°</text><text x=\"108\" y=\"128\" fill=\"#7c3aed\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">40°</text><text x=\"128\" y=\"68\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text></svg>",
              "diagramCaption": "Two chords crossing inside the circle. Not drawn to scale"
            },
            {
              "id": "cta-003",
              "topic": "Secants and Tangents",
              "difficulty": "medium",
              "stem": "Two secants intersect at a point OUTSIDE a circle. The intercepted arcs measure 120° and 30°. What is the angle formed at the intersection point?",
              "choices": [
                "90°",
                "75°",
                "15°",
                "45°"
              ],
              "correctIndex": 3,
              "explanations": [
                "90° is the difference 120° − 30° without halving. The halving step is not optional.",
                "75° halves the SUM, ½(120 + 30). The sum rule applies when the vertex is inside the circle; outside, the arcs subtract.",
                "15° halves 30°, using only the near arc. Both arcs matter — the far one carries the positive contribution.",
                "Correct. When the vertex is outside the circle, the angle is half the DIFFERENCE of the intercepted arcs: ½(120° − 30°) = ½(90°) = 45°. The far arc is the larger one, and the near arc is subtracted from it."
              ],
              "hint": "The vertex has moved outside the circle. That flips the sum in the intersecting-chords rule to a difference.",
              "diagram": "<svg viewBox=\"0 0 240 140\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><circle cx=\"150\" cy=\"70\" r=\"52\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"20\" y1=\"70\" x2=\"200\" y2=\"40\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"20\" y1=\"70\" x2=\"200\" y2=\"104\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><circle cx=\"20\" cy=\"70\" r=\"3.2\" fill=\"#c0392b\"/><text x=\"26\" y=\"62\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><text x=\"160\" y=\"26\" fill=\"#7c3aed\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">120°</text><text x=\"102\" y=\"82\" fill=\"#7c3aed\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">30°</text></svg>",
              "diagramCaption": "Both secants meet outside the circle. Not drawn to scale"
            },
            {
              "id": "cta-004",
              "topic": "Power of a Point",
              "difficulty": "medium",
              "stem": "A tangent segment of length t and a secant segment are drawn to a circle from an exterior point. The external segment of the secant is 4 and the whole secant is 16. What is t?",
              "choices": [
                "64",
                "8",
                "12",
                "10"
              ],
              "correctIndex": 1,
              "explanations": [
                "64 is t², not t. The Tangent-Secant Power Theorem gives t² = 64, and the square root still has to be taken.",
                "Correct. The Tangent-Secant Power Theorem states t² = (whole secant) × (external segment) = 16 × 4 = 64, so t = √64 = 8.",
                "12 would be the internal part of the secant, 16 − 4. That length appears in the figure but is not what the theorem multiplies — the theorem uses the WHOLE secant and its external part.",
                "10 is the average of 4 and 16. The relationship is a geometric mean, not an arithmetic one: t = √(4 × 16), not (4 + 16)/2."
              ],
              "hint": "Square the tangent, and multiply the whole secant by its outside piece. Then remember which side of the equation you were asked for.",
              "diagram": "<svg viewBox=\"0 0 240 142\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><circle cx=\"155\" cy=\"72\" r=\"48\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"22\" y1=\"96\" x2=\"122\" y2=\"38\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"22\" y1=\"96\" x2=\"210\" y2=\"82\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><circle cx=\"22\" cy=\"96\" r=\"3.2\" fill=\"#c0392b\"/><circle cx=\"122\" cy=\"38\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"63\" cy=\"92\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"203\" cy=\"78\" r=\"3.2\" fill=\"#7c3aed\"/><path d=\"M 116,44 L 124,52 L 132,44\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"56\" y=\"56\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">t</text><text x=\"30\" y=\"112\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4</text><text x=\"120\" y=\"108\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">16 (whole)</text></svg>",
              "diagramCaption": "Tangent and secant from one exterior point. Not drawn to scale"
            },
            {
              "id": "cta-005",
              "topic": "Inscribed Quadrilaterals",
              "difficulty": "medium",
              "stem": "Quadrilateral JKML is inscribed in a circle. Inscribed angle ∠K measures 135°. What is the measure of the major arc JLM that ∠K intercepts?",
              "choices": [
                "67.5°",
                "270°",
                "225°",
                "135°"
              ],
              "correctIndex": 1,
              "explanations": [
                "67.5° halves the angle instead of doubling it. Halving goes from arc to inscribed angle; here you are travelling the other way.",
                "Correct. The inscribed angle is half its intercepted arc, so the arc is twice the angle: 2(135°) = 270°. ∠K is the angle that intercepts major arc JLM, since the arc it opens onto is the one NOT containing its own vertex. The remaining arc is then 360° − 270° = 90°.",
                "225° does not follow from doubling or halving 135°. Check the arithmetic: 2 × 135 = 270.",
                "135° repeats the angle. An inscribed angle and its intercepted arc are equal only when both are zero — the arc is always twice the angle."
              ],
              "hint": "Which arc does an inscribed angle open onto — the one containing its vertex, or the other one? Then double.",
              "diagram": "<svg viewBox=\"0 0 240 146\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><circle cx=\"120\" cy=\"74\" r=\"58\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><polygon points=\"80,30 165,52 168,104 78,116\" fill=\"none\" stroke=\"#1f2024\" stroke-width=\"1.8\" stroke-linejoin=\"round\"/><circle cx=\"80\" cy=\"30\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"165\" cy=\"52\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"168\" cy=\"104\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"78\" cy=\"116\" r=\"3.2\" fill=\"#7c3aed\"/><text x=\"64\" y=\"26\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">J</text><text x=\"170\" y=\"46\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">K</text><text x=\"172\" y=\"116\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">M</text><text x=\"62\" y=\"126\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">L</text><text x=\"140\" y=\"66\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">135°</text></svg>",
              "diagramCaption": "JKML is a cyclic quadrilateral. Not drawn to scale"
            },
            {
              "id": "cta-006",
              "topic": "Inscribed Quadrilaterals",
              "difficulty": "easy",
              "stem": "A quadrilateral is inscribed in a circle. One of its angles measures 72°. What is the measure of the angle opposite it?",
              "choices": [
                "72°",
                "108°",
                "18°",
                "288°"
              ],
              "correctIndex": 1,
              "explanations": [
                "72° would make the opposite angles congruent, which is a parallelogram property, not a cyclic one. Opposite angles of an inscribed quadrilateral are supplementary instead.",
                "Correct. The Inscribed Quadrilateral Theorem says opposite angles of a cyclic quadrilateral are supplementary: 180° − 72° = 108°. The reason is that the two angles between them intercept arcs making up the whole circle, so their halves total half of 360°.",
                "18° is the complement, 90° − 72°. Complementary angles sum to 90°; these sum to 180°.",
                "288° is 360° − 72°, the reflex arc rather than an angle of the quadrilateral. No interior angle of a convex quadrilateral exceeds 180°."
              ],
              "hint": "The two arcs the opposite angles stand on together make up the entire circle. Halve 360° and see what that forces."
            },
            {
              "id": "cta-007",
              "topic": "Chords",
              "difficulty": "medium",
              "stem": "Two chords of a circle are congruent. One is labelled 6x − 7 and the other measures 35. What is the value of x?",
              "choices": [
                "7",
                "5",
                "42",
                "17.5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Congruent chords have equal length, so 6x − 7 = 35. Adding 7 gives 6x = 42, and dividing by 6 gives x = 7.",
                "5 would follow from 6x = 30, which drops the −7 or subtracts it on the wrong side. The constant has to be added to 35, not taken from it.",
                "42 is 6x, one step short. It still has to be divided by 6.",
                "17.5 is half of 35 — the half-chord you would use when dropping a perpendicular from the centre. That is a later step in a different problem, not the value of x."
              ],
              "hint": "Congruent means equal in length. Set the two expressions equal and solve — nothing circular is needed for this step."
            },
            {
              "id": "cta-008",
              "topic": "Chords",
              "difficulty": "hard",
              "stem": "A chord of length 35 lies in a circle whose centre is 12 units from the chord. What is the radius of the circle, to two decimal places?",
              "choices": [
                "21.22",
                "37.00",
                "18.50",
                "24.04"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A perpendicular dropped from the centre to a chord bisects it, giving a half-chord of 35/2 = 17.5. That half-chord, the distance 12, and the radius form a right triangle: 12² + 17.5² = r², so 144 + 306.25 = 450.25 and r = √450.25 ≈ 21.22.",
                "37.00 is 12 + 35 · … no combination of the given numbers by addition gives the radius. The relationship is Pythagorean, so the two legs must be squared before they are combined.",
                "18.50 is the half-chord 17.5 nudged, or half of 37. The half-chord is one leg of the right triangle, not the hypotenuse — the radius is longer than either leg.",
                "24.04 is √(12² + 21²) or a similar slip using the full chord somewhere it should be halved. Only HALF the chord sits in the right triangle."
              ],
              "hint": "Drop a perpendicular from the centre to the chord. It bisects the chord — so only half of 35 belongs to the right triangle you are about to solve.",
              "diagram": "<svg viewBox=\"0 0 240 160\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><circle cx=\"120\" cy=\"72\" r=\"62\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"58\" y1=\"100\" x2=\"182\" y2=\"100\" stroke=\"#1f2024\" stroke-width=\"2.2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"72\" x2=\"120\" y2=\"100\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"72\" x2=\"182\" y2=\"100\" stroke=\"#c0392b\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><path d=\"M 120,90 L 132,90 L 132,100\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><circle cx=\"120\" cy=\"72\" r=\"3.2\" fill=\"#7c3aed\"/><text x=\"126\" y=\"88\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">12</text><text x=\"140\" y=\"114\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">17.5</text><text x=\"146\" y=\"78\" fill=\"#c0392b\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">r</text><text x=\"96\" y=\"150\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">chord = 35</text></svg>",
              "diagramCaption": "The perpendicular from the centre bisects the chord. Not drawn to scale"
            },
            {
              "id": "cta-009",
              "topic": "Secants and Tangents",
              "difficulty": "medium",
              "stem": "From an exterior point H, a tangent touches a circle at G. The radius to G measures 5 cm and HG measures 8 cm. How far is H from the centre J?",
              "choices": [
                "13 cm",
                "√89 cm",
                "√39 cm",
                "40 cm"
              ],
              "correctIndex": 1,
              "explanations": [
                "13 cm adds the two lengths, 5 + 8. They are the legs of a right triangle, not collinear segments, so they combine through the Pythagorean theorem rather than by addition.",
                "Correct. By the Tangent-Radius Theorem the radius JG is perpendicular to the tangent at G, so triangle JGH has a right angle at G. Then HJ² = 8² + 5² = 64 + 25 = 89, giving HJ = √89 cm (about 9.43 cm).",
                "√39 comes from 64 − 25, subtracting the legs instead of adding them. Subtraction is right when solving for a LEG given the hypotenuse; here the hypotenuse is the unknown.",
                "40 is 5 × 8. Multiplying the legs gives twice the triangle's area, not a length."
              ],
              "hint": "What angle does a tangent make with the radius drawn to the point of tangency? That turns the figure into a right triangle.",
              "diagram": "<svg viewBox=\"0 0 240 130\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><circle cx=\"150\" cy=\"70\" r=\"42\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"25\" y1=\"88\" x2=\"128\" y2=\"40\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"128\" y1=\"40\" x2=\"150\" y2=\"70\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"88\" x2=\"150\" y2=\"70\" stroke=\"#c0392b\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><path d=\"M 122,48 L 132,54 L 126,62\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><circle cx=\"128\" cy=\"40\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"150\" cy=\"70\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"25\" cy=\"88\" r=\"3.2\" fill=\"#c0392b\"/><text x=\"14\" y=\"96\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">H</text><text x=\"122\" y=\"32\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">G</text><text x=\"156\" y=\"66\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">J</text><text x=\"60\" y=\"56\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">8 cm</text><text x=\"140\" y=\"60\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5</text><text x=\"74\" y=\"92\" fill=\"#c0392b\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text></svg>",
              "diagramCaption": "HG is tangent at G, so JG ⟂ HG. Not drawn to scale"
            },
            {
              "id": "cta-010",
              "topic": "Secants and Tangents",
              "difficulty": "hard",
              "stem": "Two tangent segments HG and HI are drawn from an exterior point H to a circle centred at J. If ∠GHJ = θ, what is m∠GHI in terms of θ, and why?",
              "choices": [
                "θ, because the tangents are congruent",
                "2θ, because HJ bisects ∠GHI",
                "90° − θ, because the radius is perpendicular to the tangent",
                "180° − θ, because GHIJ is cyclic"
              ],
              "correctIndex": 1,
              "explanations": [
                "The tangent segments ARE congruent by the Two Tangents Theorem, but that is what makes HJ a line of symmetry — it does not make the whole angle equal to half of itself.",
                "Correct. Triangles HGJ and HIJ are congruent (HG ≅ HI by the Two Tangents Theorem, JG ≅ JI as radii, HJ shared), so HJ bisects ∠GHI. The full angle is therefore twice the half-angle: m∠GHI = 2θ. On the source figure, with a radius of 5 and a tangent of 8, θ = tan⁻¹(5/8) ≈ 32.005°, giving m∠GHI ≈ 64.01°.",
                "90° − θ is the measure of ∠GJH, the angle at the CENTRE in the same right triangle — the other acute angle, not the doubled one at H.",
                "GHIJ does have supplementary opposite angles, since the two right angles at G and I put it on a circle. But that gives ∠GHI = 180° − ∠GJI, and ∠GJI is the central angle, not θ."
              ],
              "hint": "Prove the two right triangles congruent first. What does that make the segment from H to the centre?"
            },
            {
              "id": "cta-011",
              "topic": "Sector Area",
              "difficulty": "hard",
              "stem": "A sector of a circle of radius 5 has a central angle of 57.995°. What is its area, to three decimal places?",
              "choices": [
                "6.983 units²",
                "78.540 units²",
                "5.061 units²",
                "2.531 units²"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A sector is the fraction of the circle its angle cuts out: A = (θ/360)·πr² = (57.995/360)·π(5²) = (57.995/360)(25π) ≈ 6.983 units².",
                "78.540 is the area of the WHOLE circle, π(5²) ≈ 78.54. The fraction θ/360 still has to be applied.",
                "5.061 is the ARC LENGTH for this angle, (57.995/360)(2π·5) ≈ 5.06. Arc length uses the circumference; area uses πr².",
                "2.531 is half the arc length, or the arc length computed with the radius rather than the diameter. Neither is an area — check the units the formula produces."
              ],
              "hint": "A sector is a fraction of a full circle. Work out which fraction, then apply it to the whole area."
            },
            {
              "id": "cta-012",
              "topic": "Circles on the Coordinate Plane",
              "difficulty": "medium",
              "stem": "The equation x² + y² − 6x − 8y + 9 = 16 describes a circle. After completing the square, what is its radius?",
              "choices": [
                "16 units",
                "5 units",
                "3 units",
                "4 units"
              ],
              "correctIndex": 3,
              "explanations": [
                "16 is r², not r. Standard form is (x − h)² + (y − k)² = r², so the number on the right is the SQUARE of the radius.",
                "5 units would follow from r² = 25. Completing the square here gives 16 on the right, not 25 — check the constants carried across.",
                "3 units is the x-coordinate of the centre, which is 3. The centre is (3, 4); neither coordinate is the radius.",
                "Correct. Group and complete: (x² − 6x + 9) + (y² − 8y + 16) = 16 − 9 + 9 + 16, giving (x − 3)² + (y − 4)² = 16. So r² = 16 and r = √16 = 4 units, centred at (3, 4)."
              ],
              "hint": "Complete the square in x and in y separately, remembering to add the same constants to the right-hand side. Then read r² off, and take the root."
            },
            {
              "id": "cta-013",
              "topic": "Chord-Chord Power",
              "difficulty": "medium",
              "stem": "Two chords intersect inside a circle. One is divided into segments of 3 and 8; the other into segments of x and 6. Find x.",
              "choices": [
                "4",
                "5",
                "11",
                "16"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The chord–chord form of the Power of a Point says the products of the two segments of each chord are equal: 3 · 8 = x · 6, so 24 = 6x and x = 4.",
                "5 comes from subtracting rather than using products: 11 − 6 = 5. The theorem is multiplicative, not additive — segment products are what stay equal.",
                "11 is the total length of the first chord, 3 + 8. Adding the segments of one chord tells you its length but says nothing directly about the other chord's pieces.",
                "16 comes from setting 3 · 8 = x · something and dividing by 3 rather than by 6. Match each factor to the correct chord before dividing."
              ],
              "hint": "Each chord contributes a product of its two pieces. Set the two products equal."
            },
            {
              "id": "cta-014",
              "topic": "Secant-Tangent Power",
              "difficulty": "hard",
              "stem": "From an external point, a tangent segment of length t and a secant are drawn to a circle. The secant's whole length is 16 and its external portion is 4. Find t.",
              "choices": [
                "4",
                "8",
                "12",
                "64"
              ],
              "correctIndex": 1,
              "explanations": [
                "4 is the external segment of the secant, given in the problem. It is one factor in the product, not the answer.",
                "Correct. The secant–tangent form of the Power of a Point gives t² = (whole) × (external) = 16 × 4 = 64, so t = √64 = 8.",
                "12 is the INTERNAL portion of the secant, 16 − 4. The theorem multiplies the whole secant by its external part, not the internal part.",
                "64 is t², not t. The final square root was skipped — a very common last-step slip on this theorem."
              ],
              "hint": "The tangent length appears squared in this relationship. Remember to undo that at the end."
            },
            {
              "id": "cta-015",
              "topic": "External Angle",
              "difficulty": "medium",
              "stem": "Two secants drawn from an external point intercept arcs of 120° and 30°. Find the measure of the angle at the external point.",
              "choices": [
                "45°",
                "75°",
                "90°",
                "150°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. An angle formed OUTSIDE a circle by two secants equals half the DIFFERENCE of the intercepted arcs: ½(120 − 30) = ½(90) = 45°.",
                "75° is half the SUM, ½(120 + 30). Half the sum is the rule for two chords meeting INSIDE the circle. Inside uses the sum; outside uses the difference.",
                "90° is the raw difference 120 − 30 without halving. Every one of these arc-angle rules carries a factor of ½.",
                "150° is the raw sum of the two arcs, with neither the subtraction nor the halving applied."
              ],
              "hint": "Position determines the operation: vertex inside the circle behaves one way, vertex outside the other. Both then halve."
            },
            {
              "id": "cta-016",
              "topic": "Angle in a Semicircle",
              "difficulty": "easy",
              "stem": "An angle is inscribed in a semicircle. What is its measure?",
              "choices": [
                "45°",
                "60°",
                "90°",
                "180°"
              ],
              "correctIndex": 2,
              "explanations": [
                "45° would require an intercepted arc of 90°, a quarter circle. A semicircle intercepts a 180° arc.",
                "60° would require a 120° intercepted arc. That is not what a semicircle provides.",
                "Correct. An inscribed angle is half its intercepted arc, and an angle inscribed in a semicircle intercepts the full 180° arc. Half of 180° is 90°, so the angle is always right — this is Thales' theorem, and it is why a diameter subtends a right angle from any point on the circle.",
                "180° is the ARC, not the inscribed angle. Confusing an arc with the inscribed angle that intercepts it drops the factor of ½."
              ],
              "hint": "Inscribed angle equals half the intercepted arc. Work out what arc a semicircle supplies."
            },
            {
              "id": "cta-017",
              "topic": "Two Chords Inside",
              "difficulty": "medium",
              "stem": "Two chords intersect inside a circle, intercepting arcs of 80° and 40°. Find the angle formed at their intersection.",
              "choices": [
                "20°",
                "40°",
                "60°",
                "120°"
              ],
              "correctIndex": 2,
              "explanations": [
                "20° is half the DIFFERENCE, ½(80 − 40). Half the difference is the rule for a vertex OUTSIDE the circle; this vertex is inside.",
                "40° is one of the given arcs restated. The angle depends on both arcs together, not on either one alone.",
                "Correct. An angle formed by two chords meeting inside a circle equals half the SUM of the two intercepted arcs: ½(80 + 40) = ½(120) = 60°.",
                "120° is the raw sum of the arcs with the halving step omitted."
              ],
              "hint": "The vertex sits inside the circle here. That determines whether you add or subtract the arcs before halving."
            },
            {
              "id": "cta-018",
              "topic": "Reasoning: Choose the Theorem",
              "difficulty": "hard",
              "stem": "Two chords of a circle are congruent. Which conclusion follows, and by what reasoning?",
              "choices": [
                "They are parallel, because congruent chords never meet.",
                "They are equidistant from the centre, because the perpendicular distance determines chord length in a given circle.",
                "They both pass through the centre.",
                "They intercept supplementary arcs."
              ],
              "correctIndex": 1,
              "explanations": [
                "Congruent chords can certainly intersect — think of two chords of equal length crossing near the centre.",
                "Correct. In a given circle, half-chord, distance-to-centre and radius form a right triangle. With the radius fixed, equal chords force equal distances from the centre — and the converse holds too.",
                "Only a diameter passes through the centre, and it is the longest chord. Congruent chords need not be diameters.",
                "Congruent chords intercept CONGRUENT arcs, not supplementary ones."
              ],
              "hint": "Draw the perpendicular from the centre to each chord. What triangle appears, and which of its sides are fixed?",
              "kind": "reasoning"
            },
            {
              "id": "cta-019",
              "topic": "Reasoning: What's Missing",
              "difficulty": "medium",
              "stem": "A student is given that an angle formed by two lines meeting a circle measures 35°, and is asked for the arcs. Why is that not yet answerable?",
              "choices": [
                "The radius is missing.",
                "Where the vertex sits — inside, on, or outside the circle — decides which relationship applies, and it has not been stated.",
                "The angle must be a whole number of degrees.",
                "You also need to know the circumference."
              ],
              "correctIndex": 1,
              "explanations": [
                "The radius has no bearing on angle-arc relationships. They are the same in a circle of any size.",
                "Correct. A vertex at the centre gives angle = arc; on the circle, half the arc; inside, half the SUM; outside, half the DIFFERENCE. Without knowing which, the same 35° gives four different answers.",
                "35° is already a whole number, and the relationships do not require it.",
                "Circumference is a length. Arc MEASURE in degrees is independent of the circle's size."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "coordinate-geometry",
          "title": "Coordinate Geometry",
          "blurb": "Distance, midpoint, slope, writing line equations, parallel and perpendicular conditions, and partitioning a segment in a given ratio.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "coordinate-lines"
          ],
          "questions": [
            {
              "id": "cog-001",
              "topic": "Distance Formula",
              "difficulty": "easy",
              "stem": "Find the distance between the points (1, 2) and (4, 6).",
              "choices": [
                "5",
                "7",
                "√7",
                "25"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The distance formula gives d = √((4 − 1)² + (6 − 2)²) = √(3² + 4²) = √(9 + 16) = √25 = 5. The 3-4-5 right triangle is doing the work here.",
                "7 adds the two differences, 3 + 4. That is the distance you would travel going across then up — the straight-line distance is the hypotenuse of those two legs, which is shorter.",
                "√7 comes from √(4 − 1 + 6 − 2) — adding the differences before squaring. Each difference has to be squared first.",
                "25 is d², one step short. The square root still has to be taken."
              ],
              "hint": "The differences in x and y are the legs of a right triangle. The distance is its hypotenuse.",
              "diagram": "<svg viewBox=\"0 0 170 140\" width=\"100%\" style=\"max-width:170px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"30\" y1=\"-6\" x2=\"30\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"48\" y1=\"-6\" x2=\"48\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"66\" y1=\"-6\" x2=\"66\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"84\" y1=\"-6\" x2=\"84\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"102\" y1=\"-6\" x2=\"102\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"-6\" x2=\"120\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"138\" y1=\"-6\" x2=\"138\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"120\" x2=\"138\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"102\" x2=\"138\" y2=\"102\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"84\" x2=\"138\" y2=\"84\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"66\" x2=\"138\" y2=\"66\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"48\" x2=\"138\" y2=\"48\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"30\" x2=\"138\" y2=\"30\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"12\" x2=\"138\" y2=\"12\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"-6\" x2=\"138\" y2=\"-6\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"120\" x2=\"138\" y2=\"120\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"-6\" x2=\"30\" y2=\"120\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><line x1=\"48\" y1=\"84\" x2=\"102\" y2=\"12\" stroke=\"#7c3aed\" stroke-width=\"2.2\" stroke-linecap=\"round\"/><line x1=\"48\" y1=\"84\" x2=\"102\" y2=\"84\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"102\" y1=\"84\" x2=\"102\" y2=\"12\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><circle cx=\"48\" cy=\"84\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"102\" cy=\"12\" r=\"3.2\" fill=\"#7c3aed\"/><text x=\"20\" y=\"96\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">(1,2)</text><text x=\"96\" y=\"20\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">(4,6)</text><text x=\"60\" y=\"96\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3</text><text x=\"106\" y=\"62\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4</text></svg>",
              "diagramCaption": "The horizontal and vertical gaps are the legs. Not drawn to scale"
            },
            {
              "id": "cog-002",
              "topic": "Midpoint",
              "difficulty": "easy",
              "stem": "Find the midpoint of the segment joining (−3, 5) and (7, −1).",
              "choices": [
                "(2, 2)",
                "(5, 3)",
                "(4, 4)",
                "(10, −6)"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The midpoint is the average of the coordinates: ((−3 + 7)/2, (5 + (−1))/2) = (4/2, 4/2) = (2, 2).",
                "(5, 3) averages the absolute values, or halves the differences rather than the sums. The midpoint formula adds the coordinates before halving.",
                "(4, 4) is the sum of each pair without halving. Those sums are 4 and 4, which then have to be divided by 2.",
                "(10, −6) is the difference of the coordinates — the run and the rise. That is the vector from one point to the other, not the point between them."
              ],
              "hint": "A midpoint is an average. Average the x's, then average the y's."
            },
            {
              "id": "cog-003",
              "topic": "Slope",
              "difficulty": "easy",
              "stem": "Find the slope of the line through (2, 3) and (6, 11).",
              "choices": [
                "2",
                "1/2",
                "4",
                "8"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Slope is Δy/Δx = (11 − 3)/(6 − 2) = 8/4 = 2.",
                "1/2 inverts the ratio, computing Δx/Δy = 4/8. Slope is rise over run, not run over rise.",
                "4 is Δx alone, the horizontal change. Slope compares it to the vertical change.",
                "8 is Δy alone, the vertical change. It still has to be divided by the run."
              ],
              "hint": "Rise over run. Subtract in the same order on top and bottom."
            },
            {
              "id": "cog-004",
              "topic": "Equations of Lines",
              "difficulty": "medium",
              "stem": "Write the equation of the line through (0, −4) with slope 3.",
              "choices": [
                "y = 3x − 4",
                "y = −4x + 3",
                "y = 3x + 4",
                "y = 4x − 3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Point-slope gives y − y₁ = m(x − x₁), so y − (−4) = 3(x − 0), i.e. y = 3x − 4. Because the point is on the y-axis, −4 is the y-intercept and slope-intercept form can be written down directly.",
                "y = −4x + 3 swaps the slope and the intercept. The 3 is the slope and the −4 is the intercept, not the other way round.",
                "y = 3x + 4 has the right slope but the wrong sign on the intercept. The point is (0, −4), which sits below the origin.",
                "y = 4x − 3 also swaps the two numbers, keeping the signs. Check which one multiplies x."
              ],
              "hint": "The given point has x = 0, which means its y-coordinate IS the y-intercept."
            },
            {
              "id": "cog-005",
              "topic": "Parallel and Perpendicular",
              "difficulty": "medium",
              "stem": "Line A has slope 2/3. What is the slope of any line perpendicular to it?",
              "choices": [
                "2/3",
                "−3/2",
                "3/2",
                "−2/3"
              ],
              "correctIndex": 1,
              "explanations": [
                "2/3 repeats the given slope. Equal slopes mean PARALLEL lines, not perpendicular ones.",
                "Correct. Perpendicular slopes are negative reciprocals: m₁m₂ = −1, so m₂ = −1/(2/3) = −3/2. Flip the fraction and change the sign — both steps are needed.",
                "3/2 flips the fraction but keeps the sign positive. That product is (2/3)(3/2) = 1, not −1.",
                "−2/3 changes the sign but does not flip. That product is −4/9, not −1."
              ],
              "hint": "Two steps, not one: invert the fraction AND negate it. Check by multiplying — you should get −1."
            },
            {
              "id": "cog-006",
              "topic": "Parallel and Perpendicular",
              "difficulty": "medium",
              "stem": "Are the line through (0, 0) and (2, 4) and the line through (1, 1) and (3, 5) parallel?",
              "choices": [
                "Yes — both have slope 2.",
                "No — their slopes are 2 and 4.",
                "No — they intersect at the origin.",
                "Yes — they both pass through (1, 1)."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The first slope is (4 − 0)/(2 − 0) = 4/2 = 2. The second is (5 − 1)/(3 − 1) = 4/2 = 2. Equal slopes and different intercepts, so the lines are parallel.",
                "The second slope is 4/2 = 2, not 4. The rise of 4 still has to be divided by the run of 2.",
                "The first line passes through the origin; the second does not (at x = 0 it gives y = −1). And even if they did meet, that would disprove parallelism rather than explain it.",
                "The second line passes through (1, 1); the first does not — at x = 1 it gives y = 2. Sharing a point would in fact rule out parallel lines."
              ],
              "hint": "Compute both slopes. Parallel is decided by slope alone, nothing else."
            },
            {
              "id": "cog-007",
              "topic": "Distance Formula",
              "difficulty": "medium",
              "stem": "Find the length of the segment from (−2, −3) to (4, 5).",
              "choices": [
                "10",
                "14",
                "√14",
                "100"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Δx = 4 − (−2) = 6 and Δy = 5 − (−3) = 8, so d = √(36 + 64) = √100 = 10. Another 3-4-5 triangle, scaled by 2.",
                "14 adds the legs, 6 + 8. The straight-line distance is the hypotenuse, which is always shorter than going around two legs.",
                "√14 adds the differences before squaring. Square each difference first, then add.",
                "100 is d². Take the square root."
              ],
              "hint": "Watch the signs when subtracting negatives — both differences here come out positive."
            },
            {
              "id": "cog-008",
              "topic": "Midpoint",
              "difficulty": "hard",
              "stem": "A segment has midpoint (3, 4) and one endpoint (1, 2). Find the other endpoint.",
              "choices": [
                "(2, 3)",
                "(5, 6)",
                "(4, 6)",
                "(−1, 0)"
              ],
              "correctIndex": 1,
              "explanations": [
                "(2, 3) is the midpoint of (1, 2) and (3, 4) — the average of the endpoint and the midpoint. That is halfway to the halfway point.",
                "Correct. Set the midpoint formula equal to what is known: (1 + x)/2 = 3 gives x = 5, and (2 + y)/2 = 4 gives y = 6. So the other endpoint is (5, 6). Equivalently, step the same distance again: from (1,2) to (3,4) is +2 and +2, so continue to (5,6).",
                "(4, 6) has the right y but the wrong x. From 1, doubling the midpoint's 3 gives 6, minus 1 gives 5.",
                "(−1, 0) steps backwards from the known endpoint instead of forwards through the midpoint."
              ],
              "hint": "The midpoint is the average of the two ends. Write that equation with the unknown in it and solve, or just step the same distance past the midpoint."
            },
            {
              "id": "cog-009",
              "topic": "Equations of Lines",
              "difficulty": "medium",
              "stem": "Write, in slope-intercept form, the equation of the line through (2, 1) and (4, 7).",
              "choices": [
                "y = 3x − 5",
                "y = 3x + 5",
                "y = 2x − 3",
                "y = x/3 − 5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The slope is (7 − 1)/(4 − 2) = 6/2 = 3. Using point-slope at (2, 1): y − 1 = 3(x − 2), so y = 3x − 6 + 1 = 3x − 5.",
                "y = 3x + 5 has the right slope but the wrong intercept sign. Substituting x = 2 gives 11, not 1.",
                "y = 2x − 3 uses a slope of 2. The rise is 6 over a run of 2, which is 3.",
                "y = x/3 − 5 inverts the slope. Rise over run is 6/2, not 2/6."
              ],
              "hint": "Slope first, then substitute either point to pin down the intercept — and check your answer against the other point."
            },
            {
              "id": "cog-010",
              "topic": "Parallel and Perpendicular",
              "difficulty": "hard",
              "stem": "Find the equation of the line through (1, 5) that is perpendicular to y = ½x + 3.",
              "choices": [
                "y = ½x + 4.5",
                "y = −2x + 7",
                "y = 2x + 3",
                "y = −½x + 5.5"
              ],
              "correctIndex": 1,
              "explanations": [
                "This is parallel to the given line, not perpendicular — it reuses the slope ½ instead of taking the negative reciprocal.",
                "Correct. The given slope is ½, so the perpendicular slope is −2. Through (1, 5): 5 = −2(1) + b gives b = 7, so y = −2x + 7.",
                "y = 2x + 3 flips the slope but keeps it positive. The product (½)(2) = 1, not −1.",
                "y = −½x + 5.5 negates the slope without inverting it. The product (½)(−½) = −¼, not −1."
              ],
              "hint": "Negative reciprocal for the slope, then use the point to find b."
            },
            {
              "id": "cog-011",
              "topic": "Partitioning Segments",
              "difficulty": "hard",
              "stem": "Point P partitions the segment from A(0, 0) to B(10, 5) in the ratio 2:3 (measured from A). Find P.",
              "choices": [
                "(4, 2)",
                "(6, 3)",
                "(5, 2.5)",
                "(2, 3)"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A 2:3 split puts P two-fifths of the way from A to B. So P = (0 + (2/5)(10), 0 + (2/5)(5)) = (4, 2). The ratio's parts total 5, which is the denominator — not 2 or 3 on their own.",
                "(6, 3) is three-fifths of the way, which is the 3:2 split — measured from B rather than from A.",
                "(5, 2.5) is the midpoint, which would be a 1:1 ratio. A 2:3 split is not halfway.",
                "(2, 3) reads the ratio itself as coordinates. The numbers 2 and 3 describe how the segment is divided, not where the point lands."
              ],
              "hint": "A 2:3 ratio divides the segment into 5 equal parts, and P sits after 2 of them.",
              "diagram": "<svg viewBox=\"0 0 240 104\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"70\" x2=\"215\" y2=\"70\" stroke=\"#7c3aed\" stroke-width=\"2.4\" stroke-linecap=\"round\"/><circle cx=\"25\" cy=\"70\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"101\" cy=\"70\" r=\"3.2\" fill=\"#c0392b\"/><circle cx=\"215\" cy=\"70\" r=\"3.2\" fill=\"#7c3aed\"/><text x=\"16\" y=\"58\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">A(0,0)</text><text x=\"86\" y=\"58\" fill=\"#c0392b\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">P</text><text x=\"186\" y=\"58\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">B(10,5)</text><text x=\"50\" y=\"90\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">2 parts</text><text x=\"136\" y=\"90\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3 parts</text></svg>",
              "diagramCaption": "P divides AB in the ratio 2 : 3 from A. Not drawn to scale"
            },
            {
              "id": "cog-012",
              "topic": "Coordinate Proof",
              "difficulty": "medium",
              "stem": "Is the triangle with vertices (0, 0), (4, 0) and (0, 3) a right triangle?",
              "choices": [
                "Yes — the legs lie on the axes, which meet at a right angle.",
                "No — its sides are 3, 4 and 5, which is scalene.",
                "No — none of its angles is 90°.",
                "Only if the vertices are relabelled."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. One side runs along the x-axis and another along the y-axis, and the axes are perpendicular, so the angle at (0, 0) is exactly 90°. The side lengths confirm it: 3, 4 and 5, with 3² + 4² = 5².",
                "Scalene and right are not alternatives — they describe different things. A triangle is classified by its sides AND its angles, and a 3-4-5 triangle is both scalene and right.",
                "The angle at the origin is 90°, formed by the two axes. Checking the slopes gives the same answer: one side has slope 0, the other is vertical.",
                "Relabelling vertices does not change a figure's shape. The right angle is at (0, 0) however the points are named."
              ],
              "hint": "Look at where the sides lie before computing anything. Two of them are along the axes.",
              "diagram": "<svg viewBox=\"0 0 160 140\" width=\"100%\" style=\"max-width:160px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"40\" y1=\"30\" x2=\"40\" y2=\"110\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"60\" y1=\"30\" x2=\"60\" y2=\"110\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"80\" y1=\"30\" x2=\"80\" y2=\"110\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"100\" y1=\"30\" x2=\"100\" y2=\"110\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"30\" x2=\"120\" y2=\"110\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"140\" y1=\"30\" x2=\"140\" y2=\"110\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"110\" x2=\"140\" y2=\"110\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"90\" x2=\"140\" y2=\"90\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"70\" x2=\"140\" y2=\"70\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"50\" x2=\"140\" y2=\"50\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"30\" x2=\"140\" y2=\"30\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"110\" x2=\"140\" y2=\"110\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"30\" x2=\"40\" y2=\"110\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><polygon points=\"40,110 140,110 40,50\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><path d=\"M 40,98 L 52,98 L 52,110\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><circle cx=\"40\" cy=\"110\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"120\" cy=\"110\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"40\" cy=\"50\" r=\"3.2\" fill=\"#7c3aed\"/><text x=\"84\" y=\"126\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4</text><text x=\"24\" y=\"84\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3</text><text x=\"96\" y=\"74\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5</text></svg>",
              "diagramCaption": "Legs on the axes. Not drawn to scale"
            },
            {
              "id": "cog-013",
              "topic": "Distance Formula",
              "difficulty": "easy",
              "stem": "Find the distance between the points (−2, −3) and (4, 5).",
              "choices": [
                "10",
                "14",
                "√14",
                "√28"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The horizontal change is 4 − (−2) = 6 and the vertical change is 5 − (−3) = 8. Then d = √(6² + 8²) = √(36 + 64) = √100 = 10 — the 6-8-10 triple, a scaled 3-4-5.",
                "14 is 6 + 8, adding the two changes instead of combining them through the Pythagorean Theorem. That would be the walking distance along the grid, not the straight-line distance.",
                "√14 comes from adding the differences before squaring rather than squaring each first. The formula squares each difference individually, then adds.",
                "√28 comes from mishandling the negative coordinates — probably 4 − 2 = 2 and 5 − 3 = 2 instead of subtracting the negatives properly. Subtracting a negative adds."
              ],
              "hint": "Watch the signs when subtracting negative coordinates, then look for a familiar Pythagorean triple."
            },
            {
              "id": "cog-014",
              "topic": "Perpendicular Lines",
              "difficulty": "medium",
              "stem": "Find the equation of the line through (1, 5) that is perpendicular to y = ½x + 3.",
              "choices": [
                "y = ½x + 4.5",
                "y = −½x + 5.5",
                "y = −2x + 7",
                "y = 2x + 3"
              ],
              "correctIndex": 2,
              "explanations": [
                "This keeps the original slope of ½, which produces a PARALLEL line, not a perpendicular one. Parallel lines have equal slopes; perpendicular lines do not.",
                "This negates the slope but does not take the reciprocal. The perpendicular slope must satisfy m₁m₂ = −1, and (½)(−½) = −¼, not −1.",
                "Correct. The perpendicular slope is the negative reciprocal of ½, which is −2. Using point-slope through (1, 5): y − 5 = −2(x − 1), so y = −2x + 2 + 5 = −2x + 7. Check: (½)(−2) = −1.",
                "This takes the reciprocal but not the negative. A slope of 2 gives (½)(2) = 1, not −1, so the lines are not perpendicular."
              ],
              "hint": "Perpendicular slopes multiply to −1. That requires two changes to the original slope, not one."
            },
            {
              "id": "cog-015",
              "topic": "Partitioning a Segment",
              "difficulty": "hard",
              "stem": "Point P partitions the segment from A(0, 0) to B(10, 5) in the ratio 2:3, measured from A. Find P.",
              "choices": [
                "(4, 2)",
                "(6, 3)",
                "(5, 2.5)",
                "(2, 3)"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A ratio of 2:3 splits the segment into 5 equal parts, and P sits 2 of them from A — that is ⅖ of the way. So P = (0 + ⅖·10, 0 + ⅖·5) = (4, 2).",
                "(6, 3) is ⅗ of the way, which measures the 3-part from A instead of the 2-part. The ratio is read starting from the first named endpoint.",
                "(5, 2.5) is the MIDPOINT, which corresponds to a 1:1 ratio. A 2:3 split is deliberately off-center.",
                "(2, 3) uses the ratio numbers directly as coordinates. The ratio determines a fraction of the total displacement, not the coordinates themselves."
              ],
              "hint": "A ratio of a:b divides the segment into a + b equal parts. Work out what fraction of the way P sits."
            },
            {
              "id": "cog-016",
              "topic": "Slope Through Two Points",
              "difficulty": "easy",
              "stem": "Write, in slope-intercept form, the equation of the line through (2, 1) and (4, 7).",
              "choices": [
                "y = 3x − 5",
                "y = 3x + 1",
                "y = ⅓x + ⅓",
                "y = 2x − 3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The slope is (7 − 1)/(4 − 2) = 6/2 = 3. Using point (2, 1): 1 = 3(2) + b, so b = 1 − 6 = −5, giving y = 3x − 5. Check with (4, 7): 3(4) − 5 = 7. ✓",
                "The slope is right but the intercept has the wrong sign. Substituting (2,1) gives b = 1 − 6 = −5, not +1.",
                "This inverts the slope, computing run over rise instead of rise over run. Slope is the change in y divided by the change in x.",
                "A slope of 2 comes from subtracting the coordinates in mismatched order, such as (4 − 2)/(7 − 1) inverted, or from using the wrong pair of values."
              ],
              "hint": "Find the slope first, then substitute either point back to solve for the intercept — and verify with the other point."
            },
            {
              "id": "cog-017",
              "topic": "Classifying by Slope",
              "difficulty": "medium",
              "stem": "Are the line through (0, 0) and (2, 4) and the line through (1, 1) and (3, 5) parallel, perpendicular, or neither?",
              "choices": [
                "Parallel",
                "Perpendicular",
                "Neither",
                "They are the same line"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The first line has slope (4 − 0)/(2 − 0) = 2 and the second has slope (5 − 1)/(3 − 1) = 2. Equal slopes with different intercepts means the lines are parallel and never meet.",
                "Perpendicular would require the slopes to multiply to −1. Here 2 × 2 = 4, so the lines are not perpendicular.",
                "'Neither' would apply if the slopes were unequal and their product were not −1. Both slopes are exactly 2, so the parallel relationship does hold.",
                "The same line would require identical slope AND identical intercept. The first passes through the origin (b = 0) while the second gives 1 = 2(1) + b, so b = −1. Different intercepts, so they are distinct."
              ],
              "hint": "Compute both slopes. If they match, check the intercepts too before concluding the lines are merely parallel."
            },
            {
              "id": "cog-018",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "A student says two segments are perpendicular because their slopes are 3 and −3. What is the actual test, and are they perpendicular?",
              "choices": [
                "Perpendicular slopes are opposite reciprocals, so the slope would need to be −1/3. They are not perpendicular.",
                "Perpendicular slopes are simply opposite in sign, so they are perpendicular.",
                "Perpendicular slopes must multiply to 1, so they are not perpendicular.",
                "Slope cannot determine perpendicularity."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Perpendicular slopes multiply to −1, which makes them opposite RECIPROCALS. The partner for 3 is −1/3, not −3. Here 3 × (−3) = −9.",
                "Opposite signs alone are not enough — the reciprocal is the part being dropped.",
                "The product must be −1, not 1. The conclusion happens to be right, but from a wrong rule.",
                "Slope determines it perfectly well for non-vertical lines; the test is just being misremembered."
              ],
              "hint": "Multiply the two slopes. What should the product be?",
              "kind": "reasoning"
            },
            {
              "id": "cog-019",
              "topic": "Reasoning: Choose the Method",
              "difficulty": "medium",
              "stem": "To prove a quadrilateral on the coordinate plane is a rhombus, which set of calculations is sufficient?",
              "choices": [
                "The slopes of all four sides, showing opposite sides parallel.",
                "The lengths of all four sides, showing all four congruent.",
                "The midpoints of the diagonals only.",
                "The area, compared against a known rhombus."
              ],
              "correctIndex": 1,
              "explanations": [
                "Parallel opposite sides prove a PARALLELOGRAM. A rhombus needs the sides to be equal in length as well.",
                "Correct. Four congruent sides is the definition of a rhombus, and the distance formula establishes it directly.",
                "Matching diagonal midpoints proves the diagonals bisect each other, which gives a parallelogram — not enough for a rhombus.",
                "Two very different quadrilaterals can share an area, so area proves nothing about shape."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "transformations-dilations",
          "title": "Transformations & Dilations",
          "blurb": "Reflections, rotations and translations as rigid motions, plus dilations and how length, area and volume scale with k.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "congruence-transformations",
            "dilations",
            "similar-figures"
          ],
          "questions": [
            {
              "id": "trn-001",
              "topic": "Reflections",
              "difficulty": "easy",
              "stem": "Reflect the point (3, −5) over the x-axis. What are its coordinates?",
              "choices": [
                "(3, 5)",
                "(−3, −5)",
                "(−3, 5)",
                "(−5, 3)"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Reflecting over the x-axis keeps x and negates y: (x, y) → (x, −y). So (3, −5) → (3, 5).",
                "(−3, −5) negates x instead, which is a reflection over the y-axis.",
                "(−3, 5) negates both, which is a reflection through the origin (equivalently a 180° rotation).",
                "(−5, 3) swaps the coordinates and negates one — that is a 90° rotation, not a reflection."
              ],
              "hint": "The x-axis is horizontal, so reflecting across it flips the point vertically. Which coordinate measures vertical position?"
            },
            {
              "id": "trn-002",
              "topic": "Rotations",
              "difficulty": "medium",
              "stem": "Rotate the point (2, 7) by 90° counterclockwise about the origin. What are its coordinates?",
              "choices": [
                "(7, −2)",
                "(−7, 2)",
                "(−2, −7)",
                "(2, −7)"
              ],
              "correctIndex": 1,
              "explanations": [
                "(7, −2) is the 270° counterclockwise rule (equivalently 90° clockwise), (x, y) → (y, −x). It turns the point the wrong way.",
                "Correct. A 90° counterclockwise rotation about the origin is (x, y) → (−y, x). Applying it to (2, 7) gives (−7, 2).",
                "(−2, −7) is the 180° rule, (x, y) → (−x, −y). That is a half-turn, twice as far as asked.",
                "(2, −7) is a reflection over the x-axis, not a rotation. A rotation of a point not on an axis changes both coordinates here."
              ],
              "hint": "Sketch it: (2, 7) is up and slightly right. A quarter turn counterclockwise should land it up and to the LEFT."
            },
            {
              "id": "trn-003",
              "topic": "Dilations",
              "difficulty": "easy",
              "stem": "Dilate the point (4, −6) by a factor of k = ½ centred at the origin.",
              "choices": [
                "(2, −3)",
                "(8, −12)",
                "(2, −6)",
                "(3.5, −6.5)"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A dilation centred at the origin multiplies both coordinates by k: (x, y) → (kx, ky). So (4, −6) → (2, −3).",
                "(8, −12) multiplies by 2 instead of by ½. A factor below 1 shrinks the figure toward the centre.",
                "(2, −6) scales only x. A dilation acts on both coordinates, or the shape would distort rather than stay similar.",
                "(3.5, −6.5) subtracts ½ from each coordinate. Dilation multiplies; translation adds."
              ],
              "hint": "Dilation is multiplication by k, applied to both coordinates."
            },
            {
              "id": "trn-004",
              "topic": "Scale and Area",
              "difficulty": "medium",
              "stem": "A figure has area 20. After a dilation by k = 3, what is the new area?",
              "choices": [
                "60",
                "180",
                "23",
                "20"
              ],
              "correctIndex": 1,
              "explanations": [
                "60 multiplies the area by k rather than by k². Lengths scale by k, but area is a product of two lengths, so it scales by k².",
                "Correct. Area scales by the square of the factor: 20 × 3² = 20 × 9 = 180.",
                "23 adds k to the area. Dilation multiplies; it does not add.",
                "20 leaves the area unchanged. Only a rigid motion — translation, reflection, rotation — preserves area. A dilation with k ≠ 1 does not."
              ],
              "hint": "Area is length × length. If every length grows by a factor of 3, what happens to a product of two of them?"
            },
            {
              "id": "trn-005",
              "topic": "Scale and Volume",
              "difficulty": "medium",
              "stem": "A solid of volume 16 is scaled by k = 2. Find its new volume.",
              "choices": [
                "32",
                "64",
                "128",
                "18"
              ],
              "correctIndex": 2,
              "explanations": [
                "32 multiplies by k. Volume scales by k³, not k.",
                "64 multiplies by k², the area rule. Volume needs one more factor of k.",
                "Correct. Volume scales by the cube of the factor: 16 × 2³ = 16 × 8 = 128.",
                "18 adds 2. Scaling multiplies."
              ],
              "hint": "Volume is length × length × length. Count how many factors of k that gives you."
            },
            {
              "id": "trn-006",
              "topic": "Reflections",
              "difficulty": "medium",
              "stem": "Reflect the point (5, 2) over the line y = x.",
              "choices": [
                "(2, 5)",
                "(−5, −2)",
                "(5, −2)",
                "(−2, −5)"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Reflecting over y = x swaps the coordinates: (x, y) → (y, x). So (5, 2) → (2, 5). The line y = x is exactly the set of points where the two coordinates are already equal, which is why it acts as the mirror that exchanges them.",
                "(−5, −2) negates both, which is a reflection through the origin.",
                "(5, −2) reflects over the x-axis instead.",
                "(−2, −5) swaps AND negates, which is a reflection over the line y = −x."
              ],
              "hint": "Points on y = x have their coordinates equal. What does the mirror do to a point where they differ?",
              "diagram": "<svg viewBox=\"0 0 170 148\" width=\"100%\" style=\"max-width:170px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"16\" x2=\"25\" y2=\"130\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"44\" y1=\"16\" x2=\"44\" y2=\"130\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"63\" y1=\"16\" x2=\"63\" y2=\"130\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"82\" y1=\"16\" x2=\"82\" y2=\"130\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"101\" y1=\"16\" x2=\"101\" y2=\"130\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"16\" x2=\"120\" y2=\"130\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"139\" y1=\"16\" x2=\"139\" y2=\"130\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"158\" y1=\"16\" x2=\"158\" y2=\"130\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"130\" x2=\"158\" y2=\"130\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"111\" x2=\"158\" y2=\"111\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"92\" x2=\"158\" y2=\"92\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"73\" x2=\"158\" y2=\"73\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"54\" x2=\"158\" y2=\"54\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"35\" x2=\"158\" y2=\"35\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"16\" x2=\"158\" y2=\"16\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"130\" x2=\"158\" y2=\"130\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"16\" x2=\"25\" y2=\"130\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"130\" x2=\"139\" y2=\"16\" stroke=\"#6b6b76\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><circle cx=\"120\" cy=\"92\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"63\" cy=\"35\" r=\"3.2\" fill=\"#c0392b\"/><line x1=\"120\" y1=\"92\" x2=\"63\" y2=\"35\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"122\" y=\"100\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">(5,2)</text><text x=\"48\" y=\"40\" fill=\"#c0392b\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">(2,5)</text><text x=\"120\" y=\"34\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">y = x</text></svg>",
              "diagramCaption": "The mirror is the line y = x. Not drawn to scale"
            },
            {
              "id": "trn-007",
              "topic": "Rotations",
              "difficulty": "easy",
              "stem": "Rotate the point (4, 1) by 180° about the origin.",
              "choices": [
                "(−4, −1)",
                "(−1, 4)",
                "(1, −4)",
                "(4, −1)"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A 180° rotation about the origin is (x, y) → (−x, −y), so (4, 1) → (−4, −1). A half-turn sends every point to the opposite side of the origin, the same distance away.",
                "(−1, 4) is the 90° counterclockwise rule.",
                "(1, −4) is the 270° counterclockwise rule.",
                "(4, −1) reflects over the x-axis. That is not a rotation — it reverses orientation."
              ],
              "hint": "A half-turn puts the point directly opposite the centre, at the same distance."
            },
            {
              "id": "trn-008",
              "topic": "Rotations",
              "difficulty": "medium",
              "stem": "Rotate the point (3, 5) by 270° counterclockwise about the origin.",
              "choices": [
                "(−5, 3)",
                "(5, −3)",
                "(−3, −5)",
                "(3, −5)"
              ],
              "correctIndex": 1,
              "explanations": [
                "(−5, 3) is the 90° counterclockwise rule. 270° counterclockwise is three quarter-turns, which lands somewhere else.",
                "Correct. The 270° counterclockwise rule is (x, y) → (y, −x), so (3, 5) → (5, −3). Equivalently: rotate 180° to get (−3, −5), then a further 90° counterclockwise to reach (5, −3).",
                "(−3, −5) is only the 180° rotation, one quarter-turn short.",
                "(3, −5) reflects over the x-axis rather than rotating."
              ],
              "hint": "270° counterclockwise is the same as 90° clockwise. Sketch which quadrant the point should end up in."
            },
            {
              "id": "trn-009",
              "topic": "Scale and Area",
              "difficulty": "medium",
              "stem": "A rectangle of area 24 is dilated by k = ½. Find the new area.",
              "choices": [
                "12",
                "6",
                "48",
                "24"
              ],
              "correctIndex": 1,
              "explanations": [
                "12 halves the area, applying k rather than k². Halving every length quarters the area, not halves it.",
                "Correct. Area scales by k²: 24 × (½)² = 24 × ¼ = 6.",
                "48 doubles rather than shrinks. A factor below 1 makes the figure smaller.",
                "24 is unchanged, which would require k = 1."
              ],
              "hint": "Halve the width and halve the height. What fraction of the original rectangle is left?"
            },
            {
              "id": "trn-010",
              "topic": "Translations",
              "difficulty": "easy",
              "stem": "Translate the point (7, −2) by the rule (x, y) → (x − 3, y + 5).",
              "choices": [
                "(4, 3)",
                "(10, −7)",
                "(4, −7)",
                "(−3, 5)"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Apply the rule coordinate by coordinate: (7 − 3, −2 + 5) = (4, 3).",
                "(10, −7) applies both operations backwards, adding 3 and subtracting 5.",
                "(4, −7) gets x right but subtracts 5 from y instead of adding it. Watch the sign in the rule.",
                "(−3, 5) is the translation vector itself, not the image of the point. The vector says how far to move, not where you end up."
              ],
              "hint": "Substitute directly and mind the sign on the −2."
            },
            {
              "id": "trn-011",
              "topic": "Similar Solids",
              "difficulty": "hard",
              "stem": "Two similar solids have a scale factor of 2 : 3. What is the ratio of their volumes?",
              "choices": [
                "2 : 3",
                "4 : 9",
                "8 : 27",
                "6 : 9"
              ],
              "correctIndex": 2,
              "explanations": [
                "2 : 3 is the ratio of corresponding LENGTHS. Volumes grow faster than lengths.",
                "4 : 9 is the ratio of their surface AREAS, which is the scale factor squared. Volume needs the cube.",
                "Correct. Volume scales by the cube of the linear ratio: 2³ : 3³ = 8 : 27.",
                "6 : 9 multiplies both parts by 3, which reduces back to 2 : 3 — the length ratio again, just written differently."
              ],
              "hint": "Length ratio, area ratio, volume ratio — first power, second power, third power."
            },
            {
              "id": "trn-012",
              "topic": "Rigid Motions",
              "difficulty": "medium",
              "stem": "Which transformation does NOT always produce a figure congruent to the original?",
              "choices": [
                "Translation",
                "Reflection",
                "Rotation",
                "Dilation"
              ],
              "correctIndex": 3,
              "explanations": [
                "A translation slides every point the same distance in the same direction. Distances and angles are untouched, so the image is congruent.",
                "A reflection reverses orientation but preserves every distance and angle, so the image is congruent to the original.",
                "A rotation turns the figure about a fixed point, again preserving all distances and angles.",
                "Correct. A dilation multiplies every distance by k, so unless k = 1 the image is a different size. It preserves ANGLES but not lengths, which makes the image similar rather than congruent. Translation, reflection and rotation are the three rigid motions; dilation is the odd one out."
              ],
              "hint": "Congruent means same size and same shape. Which of these four changes the size?"
            },
            {
              "id": "trn-013",
              "topic": "Rotation Rules",
              "difficulty": "medium",
              "stem": "Rotate the point (3, 5) by 270° counterclockwise about the origin. What are its new coordinates?",
              "choices": [
                "(−5, 3)",
                "(5, −3)",
                "(−3, −5)",
                "(3, −5)"
              ],
              "correctIndex": 1,
              "explanations": [
                "(−5, 3) is the image under a 90° counterclockwise rotation, which uses (x, y) → (−y, x). A 270° CCW turn is three quarter-turns, landing somewhere different.",
                "Correct. A 270° counterclockwise rotation about the origin uses (x, y) → (y, −x). So (3, 5) → (5, −3). Equivalently, 270° CCW is the same as 90° clockwise.",
                "(−3, −5) is the 180° image, from (x, y) → (−x, −y). That is only two quarter-turns, not three.",
                "(3, −5) is a reflection over the x-axis, which negates y and leaves x alone. Reflections and rotations produce different images here."
              ],
              "hint": "Count quarter-turns. 270° counterclockwise is the same destination as 90° clockwise — sketch it if the rule is hard to recall."
            },
            {
              "id": "trn-014",
              "topic": "Reflection over y = x",
              "difficulty": "easy",
              "stem": "Reflect the point (5, 2) over the line y = x.",
              "choices": [
                "(−5, 2)",
                "(5, −2)",
                "(2, 5)",
                "(−2, −5)"
              ],
              "correctIndex": 2,
              "explanations": [
                "(−5, 2) is the reflection over the y-axis, which negates x. Reflecting over y = x is a different operation entirely.",
                "(5, −2) is the reflection over the x-axis, which negates y.",
                "Correct. Reflecting over the line y = x swaps the coordinates: (x, y) → (y, x). So (5, 2) → (2, 5). The line y = x is the mirror, and swapping is what mirroring across it does.",
                "(−2, −5) swaps the coordinates and then negates both, which combines a reflection over y = x with a 180° rotation. Only the swap is called for here."
              ],
              "hint": "The mirror line y = x treats the two axes symmetrically. What single operation on the coordinates reflects that symmetry?"
            },
            {
              "id": "trn-015",
              "topic": "Volume and Scale Factor",
              "difficulty": "medium",
              "stem": "A solid with volume 16 is scaled by a factor of k = 2. What is the new volume?",
              "choices": [
                "32",
                "64",
                "128",
                "48"
              ],
              "correctIndex": 2,
              "explanations": [
                "32 is 16 × 2, which scales volume by k. Only LENGTHS scale by k; volume is three-dimensional and picks up the factor three times.",
                "64 is 16 × 2², which uses the AREA scale factor. k² applies to surface areas; volume needs k³.",
                "Correct. Volume scales by k³. So the new volume is 16 × 2³ = 16 × 8 = 128.",
                "48 is 16 × 3, which multiplies by the exponent rather than raising the scale factor to it. The relationship is k³, not 3k."
              ],
              "hint": "Match the exponent on k to the number of dimensions the quantity measures."
            },
            {
              "id": "trn-016",
              "topic": "Similar Solids",
              "difficulty": "hard",
              "stem": "Two similar solids have a scale factor of 2:3. What is the ratio of their volumes?",
              "choices": [
                "2:3",
                "4:9",
                "8:27",
                "6:9"
              ],
              "correctIndex": 2,
              "explanations": [
                "2:3 is the ratio of corresponding LENGTHS — edges, radii, heights. Volume ratios are not the same as length ratios.",
                "4:9 is the ratio of SURFACE AREAS, since areas scale by k². It is the right idea applied one dimension short.",
                "Correct. Volumes of similar solids are in the ratio k³, so 2³:3³ = 8:27.",
                "6:9 multiplies both terms by 3, which simplifies right back to 2:3. Scaling both parts of a ratio equally does not change the ratio at all."
              ],
              "hint": "Cube each part of the length ratio — and check that your answer does not simplify back to the original."
            },
            {
              "id": "trn-017",
              "topic": "Rigid Motions",
              "difficulty": "medium",
              "stem": "Which transformation does NOT necessarily produce an image congruent to the original figure?",
              "choices": [
                "Translation",
                "Reflection",
                "Rotation",
                "Dilation"
              ],
              "correctIndex": 3,
              "explanations": [
                "A translation slides every point by the same vector. Distances and angles are unchanged, so it is a rigid motion and the image is congruent.",
                "A reflection flips the figure across a line. It reverses orientation but preserves all distances and angle measures, so the image is still congruent.",
                "A rotation turns the figure about a fixed point. Distances from the center are preserved and so are all internal measurements, making it rigid.",
                "Correct. A dilation multiplies all distances by the scale factor k, so unless k = ±1 the image is a different size. Angles are preserved, which makes the image SIMILAR to the original but not congruent."
              ],
              "hint": "Three of these preserve size and are called rigid motions. The fourth preserves shape only."
            },
            {
              "id": "trn-018",
              "topic": "Reasoning: Properties",
              "difficulty": "medium",
              "stem": "Which single property separates a dilation from a translation, reflection and rotation?",
              "choices": [
                "A dilation is the only one that changes orientation.",
                "A dilation is the only one that does not preserve distance, so it changes size while keeping shape.",
                "A dilation is the only one that has a fixed point.",
                "A dilation is the only one that preserves angle measure."
              ],
              "correctIndex": 1,
              "explanations": [
                "Reflection changes orientation; dilation with a positive scale factor does not.",
                "Correct. Translations, reflections and rotations are RIGID motions — they preserve every distance. A dilation scales distances by k, which changes size while preserving shape and angle measure.",
                "Rotations and reflections have fixed points too (the centre, and every point on the mirror line).",
                "All four preserve angle measure, so this cannot be what distinguishes one of them."
              ],
              "hint": "Sort the four into those that preserve distance and those that do not.",
              "kind": "reasoning"
            },
            {
              "id": "trn-019",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "hard",
              "stem": "A figure is dilated by a scale factor of 3. A student says the area is also 3 times larger. What is the correct factor, and why?",
              "choices": [
                "The area is 3 times larger; area scales the same way as length.",
                "The area is 9 times larger, because area involves two dimensions and each is scaled by 3.",
                "The area is 27 times larger, matching the cube of the scale factor.",
                "The area is unchanged; dilation preserves area."
              ],
              "correctIndex": 1,
              "explanations": [
                "Length scales by k. Area does not, because it is built from two lengths multiplied together.",
                "Correct. Area scales by k² = 9. Each of the two dimensions is stretched by 3, so the product grows by 3 × 3.",
                "k³ = 27 is how VOLUME scales, for a three-dimensional figure.",
                "A dilation with k ≠ 1 changes size by definition, so area cannot stay the same."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "pythagorean-trig",
          "title": "Pythagoras & Right-Triangle Trigonometry",
          "blurb": "The Pythagorean theorem and its triples, the 30-60-90 and 45-45-90 families, the classification test, and SOH-CAH-TOA applied to real measurements.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "pythagorean-theorem",
            "right-triangle-trig",
            "triangles-review"
          ],
          "questions": [
            {
              "id": "pyt-001",
              "topic": "Pythagorean Theorem",
              "difficulty": "easy",
              "stem": "A right triangle has legs of length 9 and 12. Find the hypotenuse.",
              "choices": [
                "15",
                "21",
                "√21",
                "225"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. c² = 9² + 12² = 81 + 144 = 225, so c = √225 = 15. This is the 3-4-5 triple scaled by 3.",
                "21 adds the legs. The hypotenuse is always shorter than the sum of the two legs — that is the Triangle Inequality.",
                "√21 adds the legs before squaring. Each leg has to be squared first.",
                "225 is c². The square root is the last step."
              ],
              "hint": "Square each leg, add, then take the root. Look for a familiar triple.",
              "diagram": "<svg viewBox=\"0 0 220 140\" width=\"100%\" style=\"max-width:220px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"30,110 190,110 30,30\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><path d=\"M 30,98 L 42,98 L 42,110\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"100\" y=\"128\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">12</text><text x=\"14\" y=\"74\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">9</text><text x=\"118\" y=\"66\" fill=\"#1f2024\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">c</text></svg>",
              "diagramCaption": "Legs 9 and 12. Not drawn to scale"
            },
            {
              "id": "pyt-002",
              "topic": "Special Right Triangles",
              "difficulty": "medium",
              "stem": "The hypotenuse of a 45-45-90 triangle is 10. Find the length of each leg.",
              "choices": [
                "5",
                "10√2",
                "5√2",
                "10/√3"
              ],
              "correctIndex": 2,
              "explanations": [
                "5 halves the hypotenuse. That is the short-leg rule for a 30-60-90 triangle, not a 45-45-90 one.",
                "10√2 multiplies by √2 instead of dividing. The legs of a 45-45-90 triangle are shorter than its hypotenuse, so multiplying by √2 ≈ 1.41 goes the wrong way.",
                "Correct. In a 45-45-90 triangle the sides are in the ratio x : x : x√2, so leg = hypotenuse/√2 = 10/√2 = 5√2 ≈ 7.07 after rationalizing.",
                "10/√3 uses the 30-60-90 ratio. The √3 belongs to that family; the 45-45-90 family uses √2."
              ],
              "hint": "The ratio is x : x : x√2. You have the x√2 part and want x.",
              "diagram": "<svg viewBox=\"0 0 200 140\" width=\"100%\" style=\"max-width:200px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"30,110 170,110 30,30\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><path d=\"M 30,98 L 42,98 L 42,110\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"120\" y=\"62\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">10</text><text x=\"88\" y=\"128\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x</text><text x=\"14\" y=\"74\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">x</text><text x=\"126\" y=\"104\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">45°</text><text x=\"36\" y=\"46\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">45°</text></svg>",
              "diagramCaption": "A 45-45-90 triangle. Not drawn to scale"
            },
            {
              "id": "pyt-003",
              "topic": "Special Right Triangles",
              "difficulty": "medium",
              "stem": "In a 30-60-90 triangle the short leg is 6. Find the long leg and the hypotenuse.",
              "choices": [
                "long leg 6√3, hypotenuse 12",
                "long leg 12, hypotenuse 6√3",
                "long leg 6√2, hypotenuse 12",
                "long leg 3√3, hypotenuse 6"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The 30-60-90 ratio is x : x√3 : 2x with x the short leg. With x = 6 the long leg is 6√3 ≈ 10.39 and the hypotenuse is 12.",
                "This swaps the two. The hypotenuse must be the longest side, and 6√3 ≈ 10.39 is less than 12 — so 12 cannot be the long leg.",
                "6√2 uses the 45-45-90 constant. The 30-60-90 family uses √3.",
                "3√3 and 6 halve the short leg first. The ratio is built FROM the short leg, so x = 6 is used directly."
              ],
              "hint": "Write the ratio x : x√3 : 2x and identify which position the given 6 occupies."
            },
            {
              "id": "pyt-004",
              "topic": "Trigonometry",
              "difficulty": "medium",
              "stem": "A 12-ft ladder leans against a wall at a 65° angle of elevation. How high up the wall does it reach? (sin 65° ≈ 0.906)",
              "choices": [
                "10.87 ft",
                "13.24 ft",
                "5.07 ft",
                "11.44 ft"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The wall height is the side opposite the 65° angle and the ladder is the hypotenuse, so sin 65° = h/12, giving h = 12(0.906) ≈ 10.87 ft.",
                "13.24 divides by the sine instead of multiplying: 12/0.906. That would make the height longer than the ladder, which is impossible.",
                "5.07 ft uses cos 65° ≈ 0.423, giving the distance from the wall along the ground — the ADJACENT side, not the height.",
                "11.44 ft does not follow from either ratio. Check which side the 65° angle is opposite."
              ],
              "hint": "SOH-CAH-TOA. The ladder is the hypotenuse; the height is opposite the angle at the ground.",
              "diagram": "<svg viewBox=\"0 0 230 138\" width=\"100%\" style=\"max-width:230px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"30\" y1=\"120\" x2=\"200\" y2=\"120\" stroke=\"#6b6b76\" stroke-width=\"1.8\" stroke-linecap=\"round\"/><line x1=\"200\" y1=\"120\" x2=\"200\" y2=\"30\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"30\" y1=\"120\" x2=\"200\" y2=\"30\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><path d=\"M 200,108 L 188,108 L 188,120\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"48\" y=\"112\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">65°</text><text x=\"96\" y=\"66\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">12 ft</text><text x=\"206\" y=\"78\" fill=\"#7c3aed\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">h</text></svg>",
              "diagramCaption": "The ladder makes 65° with the ground. Not drawn to scale"
            },
            {
              "id": "pyt-005",
              "topic": "Right Triangle Altitudes",
              "difficulty": "hard",
              "stem": "An altitude drawn to the hypotenuse of a right triangle splits it into segments of 4 and 9. Find the altitude.",
              "choices": [
                "6",
                "6.5",
                "13",
                "36"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The altitude to the hypotenuse is the geometric mean of the two segments: h = √(4 × 9) = √36 = 6.",
                "6.5 is the arithmetic mean, (4 + 9)/2. This relationship is a geometric mean — multiply and take the root.",
                "13 is the whole hypotenuse, 4 + 9.",
                "36 is h². One square root short."
              ],
              "hint": "The two small triangles are similar to each other, which gives h/4 = 9/h."
            },
            {
              "id": "pyt-006",
              "topic": "Pythagorean Theorem",
              "difficulty": "easy",
              "stem": "A right triangle has hypotenuse 13 and one leg 5. Find the other leg.",
              "choices": [
                "12",
                "18",
                "√194",
                "8"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Rearranged, b² = c² − a² = 13² − 5² = 169 − 25 = 144, so b = 12. The 5-12-13 triple.",
                "18 adds 13 and 5. Solving for a leg subtracts rather than adds, because the hypotenuse is the largest side.",
                "√194 comes from 13² + 5² = 194 — adding when you should subtract. That would give the hypotenuse of a triangle with legs 13 and 5, which is a different triangle.",
                "8 subtracts the sides directly, 13 − 5. The theorem operates on the squares, not the lengths."
              ],
              "hint": "The hypotenuse is already known, so you are solving for a leg. Which way does that rearrange the equation?"
            },
            {
              "id": "pyt-007",
              "topic": "Trigonometry",
              "difficulty": "medium",
              "stem": "In a right triangle, tan θ = 3/4 and the side opposite θ is 6. Find the adjacent side.",
              "choices": [
                "8",
                "4.5",
                "10",
                "3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. tan θ = opposite/adjacent = 3/4. With opposite = 6, solve 6/adjacent = 3/4, giving adjacent = 24/3 = 8. The triangle is a 6-8-10, the 3-4-5 scaled by 2.",
                "4.5 computes 6 × (3/4), applying the ratio in the wrong direction. Since tan θ < 1, the adjacent side must be LONGER than the opposite side.",
                "10 is the hypotenuse of this triangle, √(36 + 64). The question asks for the adjacent leg.",
                "3 repeats the numerator of the ratio. The ratio 3/4 is a proportion, not a set of lengths — the actual sides here are 6 and 8."
              ],
              "hint": "Set up the proportion 6/adjacent = 3/4 and cross-multiply. Sanity check: tan θ < 1 means adjacent > opposite."
            },
            {
              "id": "pyt-008",
              "topic": "Special Right Triangles",
              "difficulty": "medium",
              "stem": "In a 30-60-90 triangle the long leg is 5√3. Find the short leg and the hypotenuse.",
              "choices": [
                "short leg 5, hypotenuse 10",
                "short leg 5√3, hypotenuse 10√3",
                "short leg 15, hypotenuse 30",
                "short leg 5, hypotenuse 5√2"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The ratio is x : x√3 : 2x. The long leg is x√3 = 5√3, so x = 5, making the short leg 5 and the hypotenuse 2x = 10.",
                "This leaves the short leg equal to the long leg, which cannot happen in a 30-60-90 triangle — the sides opposite different angles have different lengths.",
                "15 multiplies by 3 instead of dividing by √3. Note that √3 · √3 = 3, so dividing 5√3 by √3 gives 5, not 15.",
                "5√2 is the 45-45-90 hypotenuse. In a 30-60-90 triangle the hypotenuse is exactly twice the short leg."
              ],
              "hint": "Divide the long leg by √3 to recover x, then double it for the hypotenuse."
            },
            {
              "id": "pyt-009",
              "topic": "Trigonometry",
              "difficulty": "medium",
              "stem": "From a point 50 ft from the base of a tower, the angle of elevation to the top is 40°. Find the tower's height. (tan 40° ≈ 0.839)",
              "choices": [
                "41.95 ft",
                "59.60 ft",
                "32.14 ft",
                "38.30 ft"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The 50 ft is the adjacent side and the height is opposite, so tan 40° = h/50, giving h = 50(0.839) ≈ 41.95 ft.",
                "59.60 ft divides by the tangent, 50/0.839. That would be right if the height were given and the distance sought.",
                "32.14 ft uses sin 40° ≈ 0.643. Sine relates the opposite side to the HYPOTENUSE, but 50 ft is the ground distance, which is adjacent.",
                "38.30 ft uses cos 40° ≈ 0.766, which relates the adjacent side to the hypotenuse — the wrong pairing again."
              ],
              "hint": "You have the ground distance (adjacent) and want the height (opposite). Which of the three ratios pairs exactly those two?"
            },
            {
              "id": "pyt-010",
              "topic": "Triangle Classification",
              "difficulty": "medium",
              "stem": "Classify the triangle with sides 6, 8 and 11 by its angles.",
              "choices": [
                "Right",
                "Acute",
                "Obtuse",
                "It cannot exist"
              ],
              "correctIndex": 2,
              "explanations": [
                "Right would require a² + b² = c². Here 36 + 64 = 100, which is not 121.",
                "Acute would require a² + b² > c². Here 100 < 121, so it fails in the other direction.",
                "Correct. Compare the sum of the squares of the two shorter sides with the square of the longest: 6² + 8² = 100 and 11² = 121. Since 100 < 121, the triangle is obtuse — the longest side is longer than a right angle would allow.",
                "It exists: 6 + 8 = 14 > 11, so the Triangle Inequality is satisfied. The classification test only applies once you have confirmed that."
              ],
              "hint": "Square the two shorter sides, add, and compare with the square of the longest. Less than means the angle opened past 90°.",
              "diagram": "<svg viewBox=\"0 0 240 134\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"25,105 215,105 78,42\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><text x=\"112\" y=\"124\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">11</text><text x=\"38\" y=\"68\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">6</text><text x=\"154\" y=\"66\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">8</text><text x=\"70\" y=\"60\" fill=\"#c0392b\" font-size=\"14\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text></svg>",
              "diagramCaption": "The angle opposite the longest side decides the classification. Not drawn to scale"
            },
            {
              "id": "pyt-011",
              "topic": "Special Right Triangles",
              "difficulty": "easy",
              "stem": "A 45-45-90 triangle has legs of length 7. Find the hypotenuse.",
              "choices": [
                "7√2",
                "14",
                "7√3",
                "7/√2"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The ratio x : x : x√2 gives a hypotenuse of 7√2 ≈ 9.9.",
                "14 doubles the leg. Doubling is the 30-60-90 relationship between the SHORT leg and the hypotenuse, not a 45-45-90 rule.",
                "7√3 uses the 30-60-90 constant. An isosceles right triangle carries √2.",
                "7/√2 divides instead of multiplying, giving a hypotenuse shorter than the legs — impossible, since the hypotenuse is always the longest side."
              ],
              "hint": "Both legs are 7, so Pythagoras gives c² = 49 + 49 = 98. Simplify √98."
            },
            {
              "id": "pyt-012",
              "topic": "Pythagorean Theorem",
              "difficulty": "hard",
              "stem": "Which set of three lengths is a Pythagorean triple?",
              "choices": [
                "9, 12, 16",
                "10, 24, 26",
                "5, 8, 13",
                "6, 7, 9"
              ],
              "correctIndex": 1,
              "explanations": [
                "9² + 12² = 81 + 144 = 225, but 16² = 256. Close, but not equal — 9, 12, 15 would have worked.",
                "Correct. 10² + 24² = 100 + 576 = 676, and 26² = 676. This is the 5-12-13 triple scaled by 2.",
                "5 + 8 = 13, so these three lengths do not even form a triangle — the two shorter sides collapse flat onto the longest. Certainly not a right one.",
                "6² + 7² = 36 + 49 = 85, but 9² = 81. Since 85 > 81, this triangle is acute rather than right."
              ],
              "hint": "Test a² + b² = c² on each, and watch for one that fails the Triangle Inequality before you even get that far."
            },
            {
              "id": "pyt-013",
              "topic": "Converse of Pythagoras",
              "difficulty": "medium",
              "stem": "A triangle has sides 6, 8 and 11. Classify it as right, acute, or obtuse.",
              "choices": [
                "Right",
                "Acute",
                "Obtuse",
                "No such triangle exists"
              ],
              "correctIndex": 2,
              "explanations": [
                "Right would require a² + b² = c² exactly. Here 6² + 8² = 100 while 11² = 121, so the equality fails.",
                "Acute requires a² + b² > c². Here 100 < 121, so the inequality runs the other way. The 6-8-10 triangle would be right; lengthening the longest side past 10 opens the angle beyond 90°.",
                "Correct. Compare 6² + 8² = 36 + 64 = 100 against 11² = 121. Since 100 < 121, the square of the longest side exceeds the sum of the other two squares, so the angle opposite it is obtuse.",
                "The triangle inequality is satisfied: 6 + 8 = 14 > 11, so this triangle certainly exists. It is simply obtuse."
              ],
              "hint": "Compare the sum of the squares of the two shorter sides against the square of the longest. The direction of the inequality names the triangle."
            },
            {
              "id": "pyt-014",
              "topic": "Geometric Mean",
              "difficulty": "hard",
              "stem": "In a right triangle, the altitude to the hypotenuse divides it into segments of length 4 and 9. Find the altitude.",
              "choices": [
                "6",
                "6.5",
                "13",
                "36"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The altitude to the hypotenuse is the geometric mean of the two segments it creates: h = √(p · q) = √(4 · 9) = √36 = 6.",
                "6.5 is the ARITHMETIC mean, (4 + 9)/2. The relationship here is geometric — a square root of a product, not an average of a sum.",
                "13 is the full hypotenuse, 4 + 9. That is a real length in the figure but not the altitude.",
                "36 is h², not h. The square root step was skipped."
              ],
              "hint": "The word 'mean' here is geometric, not arithmetic. That means a product under a radical."
            },
            {
              "id": "pyt-015",
              "topic": "30-60-90 Triangle",
              "difficulty": "medium",
              "stem": "A 30-60-90 triangle has a long leg of 5√3. Find the hypotenuse.",
              "choices": [
                "5",
                "10",
                "10√3",
                "5√6"
              ],
              "correctIndex": 1,
              "explanations": [
                "5 is the SHORT leg. In a 30-60-90 triangle the sides run x, x√3, 2x, so a long leg of 5√3 means x = 5 — but that is the short leg, not the hypotenuse.",
                "Correct. The ratio is x : x√3 : 2x. Matching the long leg x√3 = 5√3 gives x = 5, so the hypotenuse is 2x = 10.",
                "10√3 doubles the long leg. The hypotenuse is twice the SHORT leg, not twice the long one.",
                "5√6 comes from multiplying 5√3 by √2, which is the 45-45-90 relationship. That ratio belongs to the other special right triangle."
              ],
              "hint": "Write the three sides as multiples of x, match the one you were given, and solve for x before reading off the answer."
            },
            {
              "id": "pyt-016",
              "topic": "Angle of Elevation",
              "difficulty": "medium",
              "stem": "From a point 50 ft from the base of a tower, the angle of elevation to the top is 40°. Find the tower's height. (tan 40° ≈ 0.839)",
              "choices": [
                "41.95 ft",
                "59.6 ft",
                "32.1 ft",
                "65.3 ft"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The 50 ft is adjacent to the 40° angle and the height is opposite it, so tangent is the right ratio: tan 40° = height/50, giving height = 50(0.839) ≈ 41.95 ft.",
                "59.6 ft comes from dividing by the tangent instead of multiplying: 50/0.839. Dividing would be correct if the 50 ft were the opposite side rather than the adjacent one.",
                "32.1 ft comes from using sine instead of tangent. Sine needs the hypotenuse, and the 50 ft here is a horizontal leg on the ground, not the slanted line of sight.",
                "65.3 ft comes from using the cosine relationship inverted. Match the ratio to which two sides you actually have: opposite and adjacent means tangent."
              ],
              "hint": "Identify which sides the 50 ft and the unknown height are relative to the 40° angle, then pick the ratio that uses exactly those two."
            },
            {
              "id": "pyt-017",
              "topic": "Pythagorean Triples",
              "difficulty": "easy",
              "stem": "A right triangle has a hypotenuse of 13 and one leg of 5. Find the other leg.",
              "choices": [
                "8",
                "12",
                "12.5",
                "√194"
              ],
              "correctIndex": 1,
              "explanations": [
                "8 comes from subtracting the given lengths, 13 − 5. The Pythagorean Theorem works with squares, so lengths cannot simply be subtracted.",
                "Correct. Rearranging a² + b² = c² gives b = √(c² − a²) = √(169 − 25) = √144 = 12. This is the 5-12-13 triple, worth memorizing alongside 3-4-5.",
                "12.5 is close to the right answer but comes from an approximation rather than the exact computation. √144 is exactly 12.",
                "√194 comes from ADDING the squares, 169 + 25. Adding is correct when solving for the hypotenuse; here the hypotenuse is already known, so you subtract."
              ],
              "hint": "The hypotenuse is given, so you are solving for a leg. That changes whether you add or subtract the squares."
            },
            {
              "id": "pyt-018",
              "topic": "Reasoning: When It Applies",
              "difficulty": "medium",
              "stem": "A student applies a² + b² = c² to a triangle with sides 5, 6 and 8 to find a missing angle. Why is this invalid?",
              "choices": [
                "The Pythagorean theorem only applies to RIGHT triangles, and 5² + 6² ≠ 8².",
                "The numbers are too small.",
                "The theorem only applies when all three sides are known.",
                "It is valid; every triangle satisfies a² + b² = c²."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The theorem holds only in right triangles. Here 25 + 36 = 61 while 64 = 8², so this triangle is not right and the theorem does not apply. The Law of Cosines is the general tool.",
                "Size is irrelevant. The theorem depends on the triangle having a right angle.",
                "Knowing all three sides is fine — it is the absence of a right angle that rules the theorem out.",
                "Only right triangles satisfy it. In general a² + b² is greater than c² for acute triangles and less for obtuse ones."
              ],
              "hint": "Test the three sides against the theorem before using it. Does the equation actually hold?",
              "kind": "reasoning"
            },
            {
              "id": "pyt-019",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "hard",
              "stem": "In a right triangle, a student sets up sin θ = 8/6, where 8 is a leg and 6 is the other leg. Without computing, how do you know this is wrong?",
              "choices": [
                "Sine is opposite over HYPOTENUSE, and a ratio of two legs can exceed 1 — which sine never does.",
                "Sine only works for angles under 45°.",
                "The student should have used 6/8 instead, which is a valid sine.",
                "Nothing is wrong."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Sine is opposite ÷ hypotenuse, and the hypotenuse is always the longest side, so sine of an acute angle is always between 0 and 1. 8/6 > 1 is impossible.",
                "Sine is defined for every angle; the 45° limit is invented.",
                "6/8 is a valid number, but it is still two LEGS — that ratio is the tangent, not the sine.",
                "A sine greater than 1 is impossible for an acute angle in a right triangle."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "surface-area-volume",
          "title": "Solids: Surface Area & Volume",
          "blurb": "Prisms, cylinders, pyramids, cones and spheres — volume and surface area in both directions, plus Cavalieri's Principle.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "three-d-figures",
            "surface-area-volume"
          ],
          "questions": [
            {
              "id": "sav-001",
              "topic": "Cylinders",
              "difficulty": "easy",
              "stem": "Find the volume of a cylinder with radius 3 and height 10, in terms of π.",
              "choices": [
                "90π",
                "30π",
                "60π",
                "180π"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. V = πr²h = π(3²)(10) = π(9)(10) = 90π.",
                "30π uses πrh, forgetting to square the radius. The base is a circle of area πr².",
                "60π is the lateral surface area, 2πrh = 2π(3)(10). That is an area, not a volume.",
                "180π doubles the volume — perhaps from using 2πr²h. The prism/cylinder formula is base area times height, with no factor of 2."
              ],
              "hint": "Volume of any prism or cylinder is base area × height. What is the area of a circle of radius 3?",
              "diagram": "<svg viewBox=\"0 0 200 128\" width=\"100%\" style=\"max-width:200px; height:auto; display:block; margin:0 auto;\" role=\"img\"><ellipse cx=\"90\" cy=\"34\" rx=\"44\" ry=\"14\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"46\" y1=\"34\" x2=\"46\" y2=\"108\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"134\" y1=\"34\" x2=\"134\" y2=\"108\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><path d=\"M 46,108 A 44 14 0 0 0 134,108\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"90\" y1=\"34\" x2=\"134\" y2=\"34\" stroke=\"#1f2024\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"104\" y=\"30\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3</text><line x1=\"158\" y1=\"34\" x2=\"158\" y2=\"108\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><text x=\"164\" y=\"76\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">10</text></svg>",
              "diagramCaption": "Radius 3, height 10. Not drawn to scale"
            },
            {
              "id": "sav-002",
              "topic": "Cones",
              "difficulty": "medium",
              "stem": "Find the volume of a cone with radius 6 and height 8, in terms of π.",
              "choices": [
                "288π",
                "96π",
                "48π",
                "144π"
              ],
              "correctIndex": 1,
              "explanations": [
                "288π is the volume of the CYLINDER with the same radius and height, πr²h = π(36)(8). A cone holds exactly one third of that.",
                "Correct. V = ⅓πr²h = ⅓π(6²)(8) = ⅓π(36)(8) = 96π.",
                "48π halves rather than takes a third. The factor for a cone or pyramid is ⅓, not ½.",
                "144π uses ⅓π(36)(12) or a similar slip in the height. The height here is 8."
              ],
              "hint": "A cone is one third of the cylinder that contains it."
            },
            {
              "id": "sav-003",
              "topic": "Cones",
              "difficulty": "hard",
              "stem": "A cone has radius 6 and height 8. Find its slant height, then its lateral area (in terms of π).",
              "choices": [
                "slant 10, lateral 60π",
                "slant 10, lateral 120π",
                "slant 14, lateral 84π",
                "slant 10, lateral 96π"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The slant height is the hypotenuse of the radius and the height: ℓ = √(r² + h²) = √(36 + 64) = √100 = 10. Lateral area is πrℓ = π(6)(10) = 60π.",
                "120π doubles the lateral area, using 2πrℓ. That factor of 2 belongs to a cylinder's lateral area, not a cone's.",
                "A slant of 14 adds r and h. They are perpendicular, so they combine by Pythagoras, not by addition.",
                "96π is the cone's VOLUME, not its lateral area. Volume uses ⅓πr²h; lateral area uses πrℓ."
              ],
              "hint": "The radius, the height and the slant height form a right triangle. Find the slant first, then use πrℓ."
            },
            {
              "id": "sav-004",
              "topic": "Spheres",
              "difficulty": "medium",
              "stem": "Find the volume and surface area of a sphere with radius 3, in terms of π.",
              "choices": [
                "V = 36π, SA = 36π",
                "V = 12π, SA = 36π",
                "V = 36π, SA = 12π",
                "V = 27π, SA = 9π"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. V = (4/3)πr³ = (4/3)π(27) = 36π and SA = 4πr² = 4π(9) = 36π. They coincide numerically at r = 3, which is a coincidence of that radius rather than a general fact — the units still differ.",
                "12π for the volume drops the cube, using (4/3)πr instead. The radius is cubed for volume.",
                "12π for the surface area uses 4πr rather than 4πr². The radius is squared for area.",
                "27π is r³ times π, missing the 4/3; 9π is r² times π, missing the 4."
              ],
              "hint": "V = (4/3)πr³ and SA = 4πr². Cube for one, square for the other."
            },
            {
              "id": "sav-005",
              "topic": "Prisms",
              "difficulty": "easy",
              "stem": "A rectangular prism measures 4 × 5 × 6. Find its volume and total surface area.",
              "choices": [
                "V = 120, SA = 148",
                "V = 120, SA = 74",
                "V = 15, SA = 148",
                "V = 120, SA = 296"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. V = 4 × 5 × 6 = 120. The three distinct faces have areas 4×5 = 20, 4×6 = 24 and 5×6 = 30, and each occurs twice: SA = 2(20 + 24 + 30) = 2(74) = 148.",
                "74 is the sum of the three distinct face areas without doubling. A box has six faces, in three matching pairs.",
                "15 adds the dimensions, 4 + 5 + 6. Volume multiplies them.",
                "296 doubles the surface area twice over. Each pair is counted once in the sum, then the whole sum is doubled — once, not twice."
              ],
              "hint": "Three different faces, each appearing twice. Add the three, then double."
            },
            {
              "id": "sav-006",
              "topic": "Prisms",
              "difficulty": "easy",
              "stem": "A cube has edge length 5. Find its volume and surface area.",
              "choices": [
                "V = 125, SA = 150",
                "V = 125, SA = 125",
                "V = 15, SA = 150",
                "V = 125, SA = 25"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. V = s³ = 5³ = 125, and SA = 6s² = 6(25) = 150 — six square faces, each of area 25.",
                "125 for the surface area repeats the volume. Area and volume of a cube agree numerically only at s = 6.",
                "15 adds the three edge lengths. Volume is their product.",
                "25 is the area of ONE face. A cube has six."
              ],
              "hint": "A cube has six identical square faces."
            },
            {
              "id": "sav-007",
              "topic": "Cylinders",
              "difficulty": "medium",
              "stem": "A cylinder has volume 100π and height 4. Find its radius.",
              "choices": [
                "5",
                "25",
                "10",
                "√10"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. From V = πr²h: πr²(4) = 100π, so r² = 25 and r = 5.",
                "25 is r², one step short. Take the square root.",
                "10 would give r² = 100 and a volume of 400π — four times too large.",
                "√10 would come from r² = 10. Dividing 100 by the height 4 gives 25, not 10."
              ],
              "hint": "Divide out the π and the height first, then take a square root."
            },
            {
              "id": "sav-008",
              "topic": "Pyramids",
              "difficulty": "medium",
              "stem": "A square pyramid has base edge 6 and height 10. Find its volume.",
              "choices": [
                "360",
                "180",
                "120",
                "60"
              ],
              "correctIndex": 2,
              "explanations": [
                "360 is the volume of the PRISM with the same base and height, 36 × 10. A pyramid holds one third of it.",
                "180 halves rather than takes a third. The factor for pyramids and cones is ⅓.",
                "Correct. V = ⅓(base area)(height) = ⅓(6²)(10) = ⅓(36)(10) = 120.",
                "60 uses a base edge of 6 without squaring it, or divides by 6. The base is a 6 × 6 square of area 36."
              ],
              "hint": "Base area first — the base is a square, not an edge. Then one third of base × height."
            },
            {
              "id": "sav-009",
              "topic": "Spheres",
              "difficulty": "hard",
              "stem": "A sphere has surface area 100π. Find its radius and volume.",
              "choices": [
                "r = 5, V = 500π/3",
                "r = 25, V = 500π/3",
                "r = 5, V = 100π",
                "r = 10, V = 4000π/3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. From SA = 4πr² = 100π we get r² = 25 and r = 5. Then V = (4/3)πr³ = (4/3)π(125) = 500π/3.",
                "25 is r², not r. The square root is still needed before the volume can be computed.",
                "100π repeats the surface area as the volume. They are different quantities and, at r = 5, different numbers.",
                "r = 10 would give a surface area of 400π, four times too large."
              ],
              "hint": "Recover r from the surface area first, then feed it into the volume formula."
            },
            {
              "id": "sav-010",
              "topic": "Cones",
              "difficulty": "medium",
              "stem": "A cone has volume 12π and radius 3. Find its height.",
              "choices": [
                "4",
                "12",
                "3",
                "4/3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. From V = ⅓πr²h: ⅓π(9)h = 12π, so 3πh = 12π and h = 4.",
                "12 repeats the volume's coefficient. The height has to be extracted from the equation, not read off it.",
                "3 repeats the radius. Nothing here forces the height to equal the radius.",
                "4/3 divides by 3 one time too many. The ⅓ and the r² = 9 combine to a single factor of 3, giving 3πh = 12π."
              ],
              "hint": "Substitute what you know and simplify — the ⅓ and the 9 collapse into a single 3."
            },
            {
              "id": "sav-011",
              "topic": "Cylinders",
              "difficulty": "medium",
              "stem": "A cylinder has radius 2 and height 7. Find its total surface area, in terms of π.",
              "choices": [
                "28π",
                "36π",
                "32π",
                "44π"
              ],
              "correctIndex": 1,
              "explanations": [
                "28π is the lateral area alone, 2πrh = 2π(2)(7). The two circular ends are still missing.",
                "Correct. Total surface area is the lateral area plus the two ends: 2πrh + 2πr² = 2π(2)(7) + 2π(4) = 28π + 8π = 36π.",
                "32π adds only one end, 28π + 4π. A closed cylinder has two.",
                "44π adds the ends twice over, or uses 4πr². There are exactly two circular faces, contributing 2πr² between them."
              ],
              "hint": "Unroll it: a rectangle for the side, plus two circles for the ends."
            },
            {
              "id": "sav-012",
              "topic": "Cavalieri's Principle",
              "difficulty": "hard",
              "stem": "Two solids have the same height and the same cross-sectional area at every level. By Cavalieri's Principle, what must be true?",
              "choices": [
                "They are congruent.",
                "They have equal volumes.",
                "They have equal surface areas.",
                "They are similar."
              ],
              "correctIndex": 1,
              "explanations": [
                "Congruent means identical in size AND shape. A right cylinder and a slanted (oblique) one with the same height and radius satisfy Cavalieri's condition, yet they are clearly different shapes — so equal cross-sections cannot force congruence.",
                "Correct. Cavalieri's Principle says that if two solids have equal heights and their cross-sections at every level have equal areas, their volumes are equal. Stacking the same amount of area to the same height gives the same total, however the stack leans.",
                "Surface area is not controlled by cross-sections. Slanting a cylinder leaves its volume alone but lengthens its curved surface.",
                "Similar means the same shape at a different scale. Cavalieri's condition says nothing about shape — the two solids need not resemble each other at all."
              ],
              "hint": "Picture a straight stack of coins beside the same stack pushed into a lean. What changed, and what did not?",
              "diagram": "<svg viewBox=\"0 0 240 142\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><ellipse cx=\"60\" cy=\"30\" rx=\"30\" ry=\"10\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"30\" y1=\"30\" x2=\"30\" y2=\"110\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"90\" y1=\"30\" x2=\"90\" y2=\"110\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><path d=\"M 30,110 A 30 10 0 0 0 90,110\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><ellipse cx=\"185\" cy=\"30\" rx=\"30\" ry=\"10\" fill=\"none\" stroke=\"#1f2024\" stroke-width=\"2\"/><line x1=\"155\" y1=\"30\" x2=\"135\" y2=\"110\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"215\" y1=\"30\" x2=\"195\" y2=\"110\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><path d=\"M 135,110 A 30 10 0 0 0 195,110\" fill=\"none\" stroke=\"#1f2024\" stroke-width=\"2\"/><line x1=\"30\" y1=\"70\" x2=\"90\" y2=\"70\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"145\" y1=\"70\" x2=\"205\" y2=\"70\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"38\" y=\"132\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">right</text><text x=\"152\" y=\"132\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">oblique</text></svg>",
              "diagramCaption": "Same height, same cross-section at every level. Not drawn to scale"
            },
            {
              "id": "sav-013",
              "topic": "Cone Slant Height",
              "difficulty": "medium",
              "stem": "A cone has radius 6 and height 8. Find its lateral surface area.",
              "choices": [
                "48π",
                "60π",
                "96π",
                "100π"
              ],
              "correctIndex": 1,
              "explanations": [
                "48π uses the HEIGHT in place of the slant height: π(6)(8). Lateral area needs the slant height along the surface, not the vertical height inside.",
                "Correct. First find the slant height: ℓ = √(r² + h²) = √(36 + 64) = √100 = 10. Then lateral area = πrℓ = π(6)(10) = 60π.",
                "96π is the cone's VOLUME, ⅓π(36)(8). Volume and lateral area are different quantities with different formulas.",
                "100π is ℓ² times π — the slant height was squared instead of being multiplied by the radius."
              ],
              "hint": "Lateral area needs the slant height, which is a hypotenuse. Compute it before applying the area formula."
            },
            {
              "id": "sav-014",
              "topic": "Sphere from Surface Area",
              "difficulty": "hard",
              "stem": "A sphere has surface area 100π. Find its volume.",
              "choices": [
                "(500/3)π",
                "(100/3)π",
                "500π",
                "(250/3)π"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. From SA = 4πr² = 100π we get r² = 25, so r = 5. Then V = (4/3)πr³ = (4/3)π(125) = (500/3)π.",
                "(100/3)π comes from substituting the surface area into the volume formula in place of r³. The radius must be extracted first.",
                "500π drops the ⅓ from the sphere volume formula. The coefficient is 4/3, and forgetting the denominator is a listed high-risk error.",
                "(250/3)π uses r³ = 62.5, which comes from halving 125. Once r = 5 is found, r³ is 125 exactly."
              ],
              "hint": "Two formulas, one shared unknown. Use the surface area to pin down r, then move to the volume formula."
            },
            {
              "id": "sav-015",
              "topic": "Cylinder Radius",
              "difficulty": "medium",
              "stem": "A cylinder has volume 100π and height 4. Find its radius.",
              "choices": [
                "5",
                "25",
                "10",
                "2.5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. V = πr²h, so 100π = πr²(4). Dividing by π gives 100 = 4r², then r² = 25 and r = 5.",
                "25 is r², not r. The final square root was skipped — the same last-step slip that catches people on the tangent-secant theorem.",
                "10 comes from taking 100/(4·2.5) or otherwise mishandling the division. Divide by both π and the height before taking the root.",
                "2.5 is 10/4, which divides a length by the height. The height divides out of r², not out of r directly."
              ],
              "hint": "Strip away π and the height first to isolate r², then remember one more step remains."
            },
            {
              "id": "sav-016",
              "topic": "Cavalieri's Principle",
              "difficulty": "hard",
              "stem": "Two solids have the same height, and every horizontal cross-section at the same level has equal area. What does Cavalieri's principle conclude?",
              "choices": [
                "They are congruent.",
                "They have equal surface area.",
                "They have equal volume.",
                "They are similar with scale factor 1."
              ],
              "correctIndex": 2,
              "explanations": [
                "Congruence is far stronger than the hypothesis supports. A vertical stack of coins and the same stack pushed into a slant have matching cross-sections at every level but are not congruent.",
                "Surface areas can differ substantially. Shearing a prism leaves every cross-section unchanged while lengthening the slanted lateral faces, so the surface area grows even though the cross-sections do not.",
                "Correct. Cavalieri's principle states that if two solids have equal-area cross-sections at every level, their volumes are equal. It is what justifies the oblique-prism volume formula and underlies the derivation of the sphere's volume.",
                "Similarity with scale factor 1 would mean congruence, which the hypothesis does not give. The two solids may have entirely different shapes."
              ],
              "hint": "The principle concerns one measurement only — the one built by stacking cross-sections."
            },
            {
              "id": "sav-017",
              "topic": "Cone Height",
              "difficulty": "medium",
              "stem": "A cone has volume 12π and radius 3. Find its height.",
              "choices": [
                "4",
                "12",
                "1.33",
                "36"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. V = ⅓πr²h, so 12π = ⅓π(9)h = 3πh. Dividing by 3π gives h = 4.",
                "12 comes from omitting the ⅓ and solving 12π = π(9)h incorrectly, or from reading the volume as the height. The ⅓ factor for cones is a listed high-risk item.",
                "1.33 is ⅓ applied to the answer rather than to the formula. The ⅓ belongs on the volume side of the equation, which makes h larger, not smaller.",
                "36 comes from multiplying by 3 instead of dividing. Solve step by step: 12π = 3πh means h = 12π/3π."
              ],
              "hint": "Write out the cone volume formula including its fractional coefficient before substituting anything."
            },
            {
              "id": "sav-018",
              "topic": "Reasoning: Scaling",
              "difficulty": "hard",
              "stem": "Two cubes are similar, with edges in the ratio 2:3. What is the ratio of their volumes, and why?",
              "choices": [
                "2:3, because volume scales with edge length.",
                "4:9, matching the ratio of their surface areas.",
                "8:27, because volume involves three dimensions, each scaled by the same factor.",
                "6:9, because a cube has six faces."
              ],
              "correctIndex": 2,
              "explanations": [
                "Only LENGTHS scale by the linear ratio. Volume is built from three of them.",
                "4:9 is the ratio of the SURFACE AREAS, which scale by the square of the linear ratio.",
                "Correct. Volume scales by the cube of the linear ratio: 2³ : 3³ = 8 : 27.",
                "The number of faces does not enter into it, and it is the same for both cubes anyway."
              ],
              "hint": "Length scales by k, area by k², volume by k³. Which one is being asked for?",
              "kind": "reasoning"
            },
            {
              "id": "sav-019",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "A student reports the volume of a cylinder as 96π cm². What is wrong before any arithmetic is checked?",
              "choices": [
                "Volume cannot contain π.",
                "The units are square centimetres, but volume must be in CUBIC units — so this is an area, or the units are wrong.",
                "Volume must be a whole number.",
                "Nothing is wrong."
              ],
              "correctIndex": 1,
              "explanations": [
                "π appears in the volume of any cylinder or cone; there is nothing wrong with it.",
                "Correct. Volume is a three-dimensional measure and must be reported in cubic units — cm³. Square centimetres signal that an area formula was used, or that the units were copied carelessly.",
                "Volumes are frequently irrational, especially with π involved.",
                "The units contradict the quantity, which is exactly the kind of error a unit check is for."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "cumulative-review",
          "title": "Cumulative Mixed Practice Exam",
          "blurb": "Twelve questions jumping across the whole course — polygons, solids, right triangles, circles, coordinates and scaling. Timed like the real thing: aim for under 18 minutes, no notes.",
          "meta": "19 questions · mixed topics",
          "categories": [
            "polygons",
            "circles",
            "coordinate-lines",
            "pythagorean-theorem",
            "surface-area-volume",
            "dilations"
          ],
          "questions": [
            {
              "id": "cum-001",
              "topic": "Polygons",
              "difficulty": "medium",
              "stem": "A regular polygon has an interior angle of 144°. How many sides does it have?",
              "choices": [
                "8",
                "10",
                "12",
                "20"
              ],
              "correctIndex": 1,
              "explanations": [
                "8 sides gives an interior angle of 135°, not 144°.",
                "Correct. The exterior angle is 180° − 144° = 36°, and exterior angles total 360°, so n = 360/36 = 10.",
                "12 sides gives an interior angle of 150°.",
                "20 sides gives an interior angle of 162°."
              ],
              "hint": "Go via the exterior angle — it is the supplement, and they always add to 360°."
            },
            {
              "id": "cum-002",
              "topic": "Solids",
              "difficulty": "medium",
              "stem": "Find the volume of a cone with radius 5 and height 12, in terms of π.",
              "choices": [
                "100π",
                "300π",
                "60π",
                "200π"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. V = ⅓πr²h = ⅓π(25)(12) = ⅓π(300) = 100π.",
                "300π is the cylinder of the same dimensions. A cone is a third of it.",
                "60π uses ⅓π(5)(12) without squaring the radius.",
                "200π takes two thirds rather than one third."
              ],
              "hint": "Base area × height × ⅓."
            },
            {
              "id": "cum-003",
              "topic": "Pythagorean Theorem",
              "difficulty": "easy",
              "stem": "A right triangle has legs 8 and 15. Find the hypotenuse.",
              "choices": [
                "17",
                "23",
                "√23",
                "289"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. c² = 64 + 225 = 289, so c = 17. The 8-15-17 triple.",
                "23 adds the legs.",
                "√23 adds before squaring.",
                "289 is c²."
              ],
              "hint": "8-15-17 is one of the triples worth memorising."
            },
            {
              "id": "cum-004",
              "topic": "Circles",
              "difficulty": "medium",
              "stem": "Two chords intersect inside a circle, forming intercepted arcs of 70° and 50°. Find the angle between them.",
              "choices": [
                "60°",
                "10°",
                "120°",
                "35°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. For chords meeting inside a circle the angle is half the SUM of the intercepted arcs: ½(70 + 50) = ½(120) = 60°.",
                "10° halves the difference. The difference rule applies when the vertex is outside the circle.",
                "120° is the sum without halving.",
                "35° halves only the 70° arc. Both arcs contribute."
              ],
              "hint": "Inside the circle means add the arcs; outside means subtract. Then halve."
            },
            {
              "id": "cum-005",
              "topic": "Coordinate Geometry",
              "difficulty": "easy",
              "stem": "Find the distance between (−1, 4) and (2, 0).",
              "choices": [
                "5",
                "7",
                "√7",
                "25"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Δx = 3 and Δy = −4, so d = √(9 + 16) = √25 = 5.",
                "7 adds |Δx| and |Δy|.",
                "√7 adds the differences before squaring.",
                "25 is d²."
              ],
              "hint": "Another 3-4-5. Signs disappear once you square."
            },
            {
              "id": "cum-006",
              "topic": "Dilations",
              "difficulty": "medium",
              "stem": "A figure with area 12 is dilated by k = 4. Find the new area.",
              "choices": [
                "48",
                "192",
                "16",
                "36"
              ],
              "correctIndex": 1,
              "explanations": [
                "48 multiplies by k rather than k².",
                "Correct. Area scales by k²: 12 × 4² = 12 × 16 = 192.",
                "16 is k² alone, ignoring the original area.",
                "36 multiplies by 3. The factor is 4."
              ],
              "hint": "Area is two-dimensional, so the factor is applied twice."
            },
            {
              "id": "cum-007",
              "topic": "Triangles",
              "difficulty": "medium",
              "stem": "The angles of a triangle are x, x + 20 and 2x. Find the largest angle.",
              "choices": [
                "40°",
                "60°",
                "80°",
                "100°"
              ],
              "correctIndex": 2,
              "explanations": [
                "40° is the value of x, which is also the smallest angle. The question asks for the largest.",
                "60° is x + 20, the middle angle.",
                "Correct. The angles sum to 180°: x + (x + 20) + 2x = 4x + 20 = 180, so 4x = 160 and x = 40. The largest is 2x = 80°.",
                "100° would exceed the total once the other two are added: 40 + 60 + 100 = 200."
              ],
              "hint": "Sum to 180°, solve for x, then compare all three before answering."
            },
            {
              "id": "cum-008",
              "topic": "Special Right Triangles",
              "difficulty": "medium",
              "stem": "A 30-60-90 triangle has hypotenuse 14. Find the shorter leg.",
              "choices": [
                "7",
                "7√3",
                "14/√3",
                "7√2"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. In the ratio x : x√3 : 2x the hypotenuse is 2x, so 2x = 14 gives x = 7 as the short leg.",
                "7√3 ≈ 12.12 is the LONGER leg, x√3.",
                "14/√3 would follow from treating 14 as the long leg rather than the hypotenuse.",
                "7√2 uses the 45-45-90 constant."
              ],
              "hint": "The hypotenuse is exactly twice the short leg in a 30-60-90 triangle."
            },
            {
              "id": "cum-009",
              "topic": "Coordinate Geometry",
              "difficulty": "medium",
              "stem": "Find the equation of the line through (0, 3) perpendicular to y = −¼x.",
              "choices": [
                "y = 4x + 3",
                "y = −4x + 3",
                "y = ¼x + 3",
                "y = −¼x + 3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The negative reciprocal of −¼ is 4. Since the point is on the y-axis, the intercept is 3, giving y = 4x + 3.",
                "y = −4x + 3 inverts the slope but keeps it negative. The product (−¼)(−4) = 1, not −1.",
                "y = ¼x + 3 flips the sign without inverting. The product is −1/16.",
                "y = −¼x + 3 is parallel to the given line, not perpendicular."
              ],
              "hint": "Negative reciprocal: flip it and change the sign. The original slope is already negative."
            },
            {
              "id": "cum-010",
              "topic": "Solids",
              "difficulty": "medium",
              "stem": "A sphere has radius 6. Find its volume in terms of π.",
              "choices": [
                "144π",
                "288π",
                "216π",
                "864π"
              ],
              "correctIndex": 1,
              "explanations": [
                "144π is the surface area, 4π(36).",
                "Correct. V = (4/3)πr³ = (4/3)π(216) = 288π.",
                "216π is r³ times π, missing the 4/3.",
                "864π is 4πr³, forgetting to divide by 3."
              ],
              "hint": "Cube the radius first, then apply the 4/3."
            },
            {
              "id": "cum-011",
              "topic": "Circles",
              "difficulty": "easy",
              "stem": "An inscribed angle intercepts a 130° arc. Find the angle.",
              "choices": [
                "65°",
                "260°",
                "130°",
                "50°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. An inscribed angle is half its intercepted arc: ½(130°) = 65°.",
                "260° doubles the arc instead of halving it.",
                "130° repeats the arc — that would be the central angle on the same arc.",
                "50° is the supplement of 130° divided by nothing obvious. The rule here is simply to halve."
              ],
              "hint": "Inscribed angle equals half its arc."
            },
            {
              "id": "cum-012",
              "topic": "Area",
              "difficulty": "easy",
              "stem": "Find the area of a trapezoid with parallel sides 8 and 14 and height 5.",
              "choices": [
                "110",
                "55",
                "27",
                "70"
              ],
              "correctIndex": 1,
              "explanations": [
                "110 is (b₁ + b₂)h without halving.",
                "Correct. A = ½(b₁ + b₂)h = ½(8 + 14)(5) = ½(22)(5) = 55.",
                "27 adds all three numbers.",
                "70 multiplies only 14 by 5, using one base instead of the average of both."
              ],
              "hint": "Average the two parallel sides, then multiply by the height."
            },
            {
              "id": "cum-013",
              "topic": "Regular Polygons",
              "difficulty": "medium",
              "stem": "A regular polygon has an interior angle of 144°. How many sides does it have?",
              "choices": [
                "8",
                "10",
                "12",
                "20"
              ],
              "correctIndex": 1,
              "explanations": [
                "8 sides gives an exterior angle of 45° and an interior angle of 135°, not 144°.",
                "Correct. Interior 144° means exterior 180 − 144 = 36°. Since each exterior angle of a regular n-gon is 360/n, we get n = 360/36 = 10.",
                "12 sides gives an exterior angle of 30° and an interior angle of 150°. Close, but the arithmetic does not match 144°.",
                "20 sides comes from dividing 360 by 18 rather than 36 — likely from halving the exterior angle somewhere along the way."
              ],
              "hint": "Route through the exterior angle. It converts a fraction equation into a single division."
            },
            {
              "id": "cum-014",
              "topic": "Cone Volume",
              "difficulty": "easy",
              "stem": "Find the volume of a cone with radius 5 and height 12, in terms of π.",
              "choices": [
                "60π",
                "100π",
                "300π",
                "20π"
              ],
              "correctIndex": 1,
              "explanations": [
                "60π multiplies the radius and height without squaring the radius or applying the ⅓. The base is a circle, so r must be squared.",
                "Correct. V = ⅓πr²h = ⅓π(25)(12) = ⅓π(300) = 100π.",
                "300π is the CYLINDER volume πr²h with the same dimensions. A cone holds exactly one third of the cylinder that contains it — forgetting the ⅓ is a listed high-risk error.",
                "20π comes from using the diameter or otherwise mis-substituting. Substitute r = 5 and square it before multiplying."
              ],
              "hint": "A cone is a fraction of the cylinder with the same base and height. Do not lose that fraction."
            },
            {
              "id": "cum-015",
              "topic": "Special Right Triangles",
              "difficulty": "medium",
              "stem": "A 30-60-90 triangle has a hypotenuse of 14. Find the shorter leg.",
              "choices": [
                "7",
                "7√3",
                "14√3",
                "7√2"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. In a 30-60-90 triangle the sides are x : x√3 : 2x, so the hypotenuse is twice the short leg. From 2x = 14 we get x = 7.",
                "7√3 is the LONG leg, x√3 with x = 7. It sits opposite the 60° angle, not the 30° one.",
                "14√3 multiplies the hypotenuse by √3, which is not part of this ratio. The √3 attaches to the short leg to produce the long leg.",
                "7√2 uses the 45-45-90 relationship, where the hypotenuse is a leg times √2. That is the other special right triangle."
              ],
              "hint": "In this ratio the hypotenuse has the simplest relationship to one particular leg. Identify which."
            },
            {
              "id": "cum-016",
              "topic": "Mixed Review",
              "difficulty": "medium",
              "stem": "The angles of a triangle measure x, x + 20, and 2x. Find the largest angle.",
              "choices": [
                "40°",
                "60°",
                "80°",
                "100°"
              ],
              "correctIndex": 2,
              "explanations": [
                "40° is the value of x and the smallest of the three angles. Solving x + (x + 20) + 2x = 180 gives 4x = 160 and x = 40, but the question asks for the largest.",
                "60° is the middle angle, x + 20 = 40 + 20. The three angles are 40°, 60° and 80°, so this is not the largest.",
                "Correct. The angle sum gives x + (x + 20) + 2x = 180, so 4x + 20 = 180, then 4x = 160 and x = 40. The angles are 40°, 60° and 80°, making 80° the largest.",
                "100° would break the angle sum: 40 + 60 + 100 = 200, which exceeds 180°. No triangle has these angles."
              ],
              "hint": "Solve for the variable, evaluate all three expressions, and only then compare."
            },
            {
              "id": "cum-017",
              "topic": "Trapezoid Area",
              "difficulty": "easy",
              "stem": "Find the area of a trapezoid with bases 8 and 14 and height 5.",
              "choices": [
                "55",
                "110",
                "70",
                "27"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A = ½(b₁ + b₂)h = ½(8 + 14)(5) = ½(22)(5) = 55. The formula averages the two bases and multiplies by the height.",
                "110 is (8 + 14)(5) with the ½ omitted. Without halving you get the area of a parallelogram whose base is the sum of both — twice too much.",
                "70 is 14 × 5, using only the longer base. A trapezoid's bases differ, so both must enter the calculation.",
                "27 adds the three given numbers, 8 + 14 + 5. Area is a product-based quantity, not a sum of the given measurements."
              ],
              "hint": "The trapezoid formula takes the average of the two parallel sides, then treats that as a single base."
            },
            {
              "id": "cum-018",
              "topic": "Reasoning: Strategy",
              "difficulty": "hard",
              "stem": "You must find a side of a triangle. You know two sides and the angle BETWEEN them. Which tool applies?",
              "choices": [
                "The Pythagorean theorem, since two sides are known.",
                "The Law of Cosines, which handles two sides and their included angle.",
                "The Law of Sines, since an angle is known.",
                "Not enough information."
              ],
              "correctIndex": 1,
              "explanations": [
                "The Pythagorean theorem needs a RIGHT angle. Nothing says the included angle is 90°.",
                "Correct. Two sides and the included angle (SAS) is exactly the Law of Cosines case: c² = a² + b² − 2ab·cos C.",
                "The Law of Sines needs an angle paired with its OPPOSITE side. Here the known angle sits between the known sides.",
                "SAS determines a triangle completely, so the third side is fully determined."
              ],
              "hint": "Ask which parts are paired. The Law of Sines needs an angle and the side across from it.",
              "kind": "reasoning"
            },
            {
              "id": "cum-019",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "A student proves two triangles congruent by AAA and concludes the triangles are the same size. What is wrong?",
              "choices": [
                "AAA is not a congruence criterion — it establishes SIMILARITY, which fixes shape but not size.",
                "AAA is valid; the student made an arithmetic error.",
                "AAA only works for right triangles.",
                "The student should have written AAS, which means the same thing."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Three pairs of congruent angles guarantee the same SHAPE but say nothing about size — a small triangle and a large one can have identical angles. Congruence needs at least one side: SSS, SAS, ASA, AAS or HL.",
                "AAA is not a congruence criterion at all, so there is no arithmetic that would rescue it.",
                "AAA fails to prove congruence for right triangles too. HL is the right-triangle shortcut.",
                "AAS includes a SIDE, which is exactly what AAA lacks. They are not the same."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "congruence-similarity",
          "title": "Congruence, Similarity & Symmetry",
          "blurb": "The five congruence criteria and why SSA is not one of them, CPCTC, the similarity criteria and how area scales, plus lines and orders of symmetry.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "congruent-triangles",
            "similar-figures",
            "congruent-figures-review",
            "symmetry"
          ],
          "questions": [
            {
              "id": "cgs-001",
              "topic": "Congruence Criteria",
              "difficulty": "medium",
              "stem": "Which triangle congruence shortcut is INVALID, because it can produce two different triangles from the same data?",
              "choices": [
                "SSA",
                "HL",
                "AAS",
                "SAS"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Side-Side-Angle does not fix a triangle. With two sides and an angle NOT between them, the third vertex can often be placed two different ways — the swinging side reaches the base line at two points. This is the ambiguous case, and it is why SSA is not a congruence criterion.",
                "HL is valid, but only for right triangles: a congruent hypotenuse and one congruent leg force the third side by the Pythagorean theorem, so it is really SSS in disguise.",
                "AAS is valid. Two angles fix the third (they sum to 180°), so AAS becomes ASA once you name the third angle.",
                "SAS is valid. The angle sits BETWEEN the two sides, which locks the third vertex in exactly one place — the difference between SAS and SSA is entirely about where the angle is."
              ],
              "hint": "Try to build a triangle from two sides and a non-included angle. Can the free side swing to meet the base in more than one spot?",
              "diagram": "<svg viewBox=\"0 0 240 138\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"25\" y1=\"105\" x2=\"215\" y2=\"105\" stroke=\"#6b6b76\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><line x1=\"25\" y1=\"105\" x2=\"130\" y2=\"30\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"130\" y1=\"30\" x2=\"90\" y2=\"105\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"130\" y1=\"30\" x2=\"185\" y2=\"105\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><circle cx=\"25\" cy=\"105\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"130\" cy=\"30\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"90\" cy=\"105\" r=\"3.2\" fill=\"#1f2024\"/><circle cx=\"185\" cy=\"105\" r=\"3.2\" fill=\"#c0392b\"/><text x=\"38\" y=\"98\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">θ</text><text x=\"66\" y=\"62\" fill=\"#7c3aed\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">a</text><text x=\"108\" y=\"74\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">b</text><text x=\"166\" y=\"66\" fill=\"#c0392b\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">b</text><text x=\"72\" y=\"128\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">two possible triangles</text></svg>",
              "diagramCaption": "The same two sides and non-included angle close two different ways. Not drawn to scale"
            },
            {
              "id": "cgs-002",
              "topic": "Congruence Criteria",
              "difficulty": "easy",
              "stem": "What does the abbreviation CPCTC stand for in geometric proofs?",
              "choices": [
                "Corresponding Parts of Congruent Triangles are Congruent",
                "Corresponding Parts of Central Triangles are Congruent",
                "Calculated Proportions of Congruent Triangles create Congruence",
                "Congruent Parts of Congruent Triangles are Congruent"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Once two triangles are proved congruent, every one of the six corresponding parts — three sides and three angles — is congruent. It is the standard closing move of a two-stage proof: establish congruence by a criterion, then quote CPCTC to collect the specific part you wanted.",
                "\"Central\" has no meaning here. The word is \"Corresponding\", which names the matching-up of vertices that the congruence statement declares.",
                "Nothing is calculated and no proportions are involved — proportions belong to SIMILAR triangles, not congruent ones.",
                "This says congruent parts are congruent, which is circular and useless. The content of CPCTC is that CORRESPONDING parts become congruent once the triangles are."
              ],
              "hint": "Two of these four differ by one word. Which word describes how the vertices are matched up?"
            },
            {
              "id": "cgs-003",
              "topic": "Similar Figures",
              "difficulty": "medium",
              "stem": "Two similar triangles have a side-length scale factor of 2 : 5. What is the ratio of their areas?",
              "choices": [
                "2 : 5",
                "4 : 25",
                "8 : 125",
                "4 : 10"
              ],
              "correctIndex": 1,
              "explanations": [
                "2 : 5 is the ratio of LENGTHS. Areas grow faster, because an area is a product of two lengths.",
                "Correct. The ratio of areas is the square of the ratio of lengths: 2² : 5² = 4 : 25.",
                "8 : 125 is the cube, which is the ratio of VOLUMES for similar solids. This question is about a flat figure.",
                "4 : 10 doubles both parts, which reduces straight back to 2 : 5 — the length ratio again."
              ],
              "hint": "Length ratio, area ratio, volume ratio: first power, second power, third power."
            },
            {
              "id": "cgs-004",
              "topic": "Similarity Criteria",
              "difficulty": "medium",
              "stem": "Which condition is enough on its own to prove two triangles SIMILAR?",
              "choices": [
                "Two pairs of congruent angles (AA)",
                "Two pairs of congruent sides (SS)",
                "Three pairs of congruent angles AND one pair of congruent sides",
                "Any pair of congruent angles"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. AA is enough. If two angles match, the third must match too (all three sum to 180°), and equal angles force the sides into proportion. Similarity needs only shape, not size — which is exactly why it takes less evidence than congruence.",
                "Two pairs of congruent sides says nothing without knowing the angle between them or the third side. The similarity version is SAS~, which needs two pairs of PROPORTIONAL sides and the included angle.",
                "This is more than enough — in fact a congruent side alongside three congruent angles proves the triangles CONGRUENT, which is stronger than similar. The question asks for what suffices, and AA already does.",
                "One pair of congruent angles is not enough. Two triangles can share a 40° angle and be nothing alike."
              ],
              "hint": "How many angles do you need before the third is determined for free?"
            },
            {
              "id": "cgs-005",
              "topic": "Congruence Criteria",
              "difficulty": "medium",
              "stem": "Two right triangles have congruent hypotenuses and one pair of congruent legs. Which criterion proves them congruent?",
              "choices": [
                "HL",
                "SSA",
                "AAA",
                "They cannot be proved congruent"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Hypotenuse-Leg applies only to right triangles, and it works because the right angle plus the Pythagorean theorem force the remaining leg: if the hypotenuse and one leg match, the other leg must too. HL is SSS with the third side computed rather than measured.",
                "The data here IS side-side-angle in form — two sides and a non-included right angle — which is normally ambiguous. The right angle is the special case that removes the ambiguity, and HL is the name for that case.",
                "AAA proves similarity, never congruence. Three matching angles fix the shape but say nothing about size.",
                "They certainly can. HL is one of the five standard criteria precisely for this configuration."
              ],
              "hint": "A right triangle's third side is never independent. What determines it once you know the other two?"
            },
            {
              "id": "cgs-006",
              "topic": "Congruent Figures",
              "difficulty": "easy",
              "stem": "Figure B was produced from figure A by a translation followed by a reflection. What is the relationship between A and B?",
              "choices": [
                "Congruent",
                "Similar but not congruent",
                "Neither similar nor congruent",
                "Congruent only if the reflection was over an axis"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Translations and reflections are both rigid motions — they preserve every distance and angle — and composing rigid motions gives another rigid motion. So B is congruent to A. Its orientation is reversed by the reflection, but reversed orientation does not affect congruence.",
                "Similar-but-not-congruent requires a size change, which only a dilation with k ≠ 1 produces. Neither transformation here changes size.",
                "Every rigid motion preserves both shape and size, so the figures cannot fail to be at least congruent.",
                "The line of reflection makes no difference. Reflecting over any line — axis, diagonal, or otherwise — preserves distances."
              ],
              "hint": "Sort the four transformations into those that change size and those that do not. Which kind were both of these?"
            },
            {
              "id": "cgs-007",
              "topic": "Congruent Figures",
              "difficulty": "medium",
              "stem": "Triangle ABC maps onto triangle DEF under a sequence of rigid motions, with A → D, B → E, C → F. Which statement must be true?",
              "choices": [
                "∠A ≅ ∠D and AB ≅ DE",
                "∠A ≅ ∠E and AB ≅ EF",
                "AB ≅ DF only",
                "The triangles have equal perimeters but possibly different angles"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The correspondence stated by the mapping is what tells you which parts match: A pairs with D, so ∠A ≅ ∠D, and side AB pairs with side DE. Reading the correspondence off the statement is the whole skill here.",
                "This pairs A with E, contradicting the given mapping. A congruence statement is an ordered claim — the order of the letters IS the correspondence.",
                "AB corresponds to DE, not DF. And it is not the only congruent pair: all six corresponding parts are congruent.",
                "Rigid motions preserve angles as well as lengths, so the angles match too. Equal perimeters with different angles would describe two figures that are not congruent at all."
              ],
              "hint": "Read the letters in the order given. That order is not decoration — it names which part matches which."
            },
            {
              "id": "cgs-008",
              "topic": "Symmetry",
              "difficulty": "easy",
              "stem": "How many lines of symmetry does a regular hexagon have?",
              "choices": [
                "3",
                "6",
                "12",
                "0"
              ],
              "correctIndex": 1,
              "explanations": [
                "3 counts only the lines through opposite vertices. There are three more through the midpoints of opposite sides.",
                "Correct. A regular n-gon has exactly n lines of symmetry, so a regular hexagon has 6: three joining opposite vertices, and three joining the midpoints of opposite sides.",
                "12 is the size of the hexagon's full symmetry group — 6 reflections plus 6 rotations. The question asks only for the reflection lines.",
                "0 would describe a figure with no mirror symmetry at all, like a general scalene triangle. A regular hexagon is as symmetric as a six-sided figure gets."
              ],
              "hint": "Two kinds of mirror line are available: through opposite corners, and through opposite edge midpoints. Count both.",
              "diagram": "<svg viewBox=\"0 0 240 162\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"120,15 178,48 178,114 120,147 62,114 62,48\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"120\" y1=\"15\" x2=\"120\" y2=\"147\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"178\" y1=\"48\" x2=\"62\" y2=\"114\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"178\" y1=\"114\" x2=\"62\" y2=\"48\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"90\" y1=\"31\" x2=\"150\" y2=\"131\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"150\" y1=\"31\" x2=\"90\" y2=\"131\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"62\" y1=\"81\" x2=\"178\" y2=\"81\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/></svg>",
              "diagramCaption": "Three through opposite vertices, three through opposite side midpoints. Not drawn to scale"
            },
            {
              "id": "cgs-009",
              "topic": "Symmetry",
              "difficulty": "medium",
              "stem": "A parallelogram that is not a rectangle or a rhombus has how many lines of symmetry, and what order of rotational symmetry?",
              "choices": [
                "0 lines, order 2",
                "2 lines, order 2",
                "1 line, order 1",
                "0 lines, order 1"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A general parallelogram has NO line of symmetry — folding it along either diagonal or either midline does not match the halves up. But it does have rotational symmetry of order 2: turning it 180° about the intersection of its diagonals maps it onto itself.",
                "2 lines would describe a rectangle or a rhombus. Adding right angles or equal sides is exactly what buys the mirror lines.",
                "1 line describes an isosceles trapezoid or a kite. A parallelogram has none.",
                "Order 1 means no rotational symmetry beyond a full turn. A parallelogram genuinely does map onto itself at 180°, which is order 2."
              ],
              "hint": "Try folding a leaning parallelogram in half — any way you like. Then try spinning it half a turn."
            },
            {
              "id": "cgs-010",
              "topic": "Symmetry",
              "difficulty": "medium",
              "stem": "A figure maps onto itself after a rotation of 72° about its centre. What is the smallest number of sides it could have as a regular polygon?",
              "choices": [
                "4",
                "5",
                "6",
                "72"
              ],
              "correctIndex": 1,
              "explanations": [
                "A square maps onto itself every 90°, not every 72°.",
                "Correct. A regular n-gon maps onto itself every 360°/n. Setting 360/n = 72 gives n = 5 — a regular pentagon.",
                "A regular hexagon has a 60° rotational symmetry, not 72°.",
                "72 is the angle in degrees, not a count of sides. A regular 72-gon would map onto itself every 5°."
              ],
              "hint": "The smallest rotation that works for a regular n-gon is one nth of a full turn."
            },
            {
              "id": "cgs-011",
              "topic": "Similar Figures",
              "difficulty": "hard",
              "stem": "In △ABC, a line parallel to BC cuts AB at D and AC at E. If AD = 4, DB = 6 and AE = 6, what is EC?",
              "choices": [
                "4",
                "9",
                "10",
                "6"
              ],
              "correctIndex": 1,
              "explanations": [
                "4 repeats AD. The two sides are cut in the same RATIO, not to the same lengths.",
                "Correct. A line parallel to one side cuts the other two proportionally (the Side-Splitter Theorem, which follows from △ADE ~ △ABC by AA). So AD/DB = AE/EC gives 4/6 = 6/EC, and EC = 36/4 = 9.",
                "10 would make AC = 16 and AE/EC = 6/10 = 3/5, which is not 4/6 = 2/3.",
                "6 repeats AE. It would make the split 6:6 on one side and 4:6 on the other — not proportional."
              ],
              "hint": "The parallel line creates a smaller triangle similar to the whole. Set the two side splits into a proportion.",
              "diagram": "<svg viewBox=\"0 0 240 148\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"120,20 30,125 210,125\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"72\" y1=\"76\" x2=\"168\" y2=\"76\" stroke=\"#1f2024\" stroke-width=\"2\" stroke-linecap=\"round\"/><circle cx=\"72\" cy=\"76\" r=\"3.2\" fill=\"#1f2024\"/><circle cx=\"168\" cy=\"76\" r=\"3.2\" fill=\"#1f2024\"/><text x=\"112\" y=\"14\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">A</text><text x=\"20\" y=\"138\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">B</text><text x=\"212\" y=\"138\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">C</text><text x=\"56\" y=\"74\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">D</text><text x=\"174\" y=\"74\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">E</text><text x=\"86\" y=\"50\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4</text><text x=\"50\" y=\"106\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">6</text><text x=\"150\" y=\"50\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">6</text><text x=\"186\" y=\"106\" fill=\"#c0392b\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text></svg>",
              "diagramCaption": "DE ∥ BC. Not drawn to scale"
            },
            {
              "id": "cgs-012",
              "topic": "Similar Figures",
              "difficulty": "medium",
              "stem": "Two regular pentagons have perimeters of 30 cm and 45 cm. What is the ratio of their areas?",
              "choices": [
                "2 : 3",
                "4 : 9",
                "8 : 27",
                "30 : 45"
              ],
              "correctIndex": 1,
              "explanations": [
                "2 : 3 is the ratio of perimeters (and so of side lengths), reduced. Areas scale by the square of that.",
                "Correct. All regular pentagons are similar, so the perimeter ratio 30 : 45 = 2 : 3 is the length ratio, and the area ratio is 2² : 3² = 4 : 9.",
                "8 : 27 is the cube, which would be the volume ratio for similar solids. These are flat figures.",
                "30 : 45 is the unreduced perimeter ratio, which is the LENGTH ratio, not the area ratio."
              ],
              "hint": "Perimeter is a length, so the perimeter ratio is the length ratio. Now square it."
            },
            {
              "id": "cgs-013",
              "topic": "Invalid Congruence Criteria",
              "difficulty": "medium",
              "stem": "Which of the following is NOT a valid triangle congruence criterion?",
              "choices": [
                "SAS",
                "ASA",
                "SSA",
                "HL"
              ],
              "correctIndex": 2,
              "explanations": [
                "SAS is valid: two sides and the angle BETWEEN them fix the triangle completely, because the included angle locks the two sides into one configuration.",
                "ASA is valid: two angles and the included side determine the third angle by the angle sum, and from there the whole triangle.",
                "Correct. SSA is not a valid criterion. With two sides and a non-included angle, the third side can sometimes swing to two different positions, producing two non-congruent triangles — the ambiguous case that returns in the Law of Sines.",
                "HL is valid, but only for RIGHT triangles: hypotenuse and one leg determine the other leg by the Pythagorean Theorem, which reduces it to SSS."
              ],
              "hint": "One of these leaves room for two different triangles from the same data. That ambiguity is what disqualifies it."
            },
            {
              "id": "cgs-014",
              "topic": "Midsegment Theorem",
              "difficulty": "easy",
              "stem": "A midsegment of a triangle measures 7 cm. How long is the side it is parallel to?",
              "choices": [
                "3.5 cm",
                "7 cm",
                "14 cm",
                "21 cm"
              ],
              "correctIndex": 2,
              "explanations": [
                "3.5 cm halves the midsegment. The relationship runs the other way: the midsegment is half the side, so the side is the larger of the two.",
                "7 cm would make the midsegment equal to the side it parallels, which never happens in a genuine triangle.",
                "Correct. The Midsegment Theorem says a segment joining the midpoints of two sides is parallel to the third side and half its length. So the third side is 2 × 7 = 14 cm.",
                "21 cm triples the midsegment. The factor in the Midsegment Theorem is 2, not 3."
              ],
              "hint": "Decide which of the two segments the theorem describes as 'half'. That tells you whether to double or halve."
            },
            {
              "id": "cgs-015",
              "topic": "Similar Triangles",
              "difficulty": "medium",
              "stem": "△PQR ~ △XYZ with PQ = 12 and XY = 8. If the perimeter of △XYZ is 30, find the perimeter of △PQR.",
              "choices": [
                "20",
                "45",
                "50",
                "67.5"
              ],
              "correctIndex": 1,
              "explanations": [
                "20 scales in the wrong direction, multiplying by 8/12 instead of 12/8. Since PQ is longer than XY, △PQR is the larger triangle and its perimeter must exceed 30.",
                "Correct. The scale factor from XYZ to PQR is 12/8 = 1.5. Perimeter is a length measurement, so it scales by the same factor: 30 × 1.5 = 45.",
                "50 does not follow from any consistent scaling of 30. Apply the ratio directly rather than estimating.",
                "67.5 applies the scale factor twice, 30 × 1.5². Squaring the factor is correct for AREA, not for perimeter — perimeter is one-dimensional."
              ],
              "hint": "Perimeter is a sum of lengths, so it scales the same way a single length does."
            },
            {
              "id": "cgs-016",
              "topic": "AA Similarity",
              "difficulty": "easy",
              "stem": "Two triangles have two pairs of congruent angles. What can be concluded?",
              "choices": [
                "They are congruent.",
                "They are similar.",
                "Nothing — two angles is not enough.",
                "They are congruent only if a side is also known."
              ],
              "correctIndex": 1,
              "explanations": [
                "Congruence requires matching SIZE as well as shape. Two triangles can have identical angles and wildly different sizes — think of a photograph and its enlargement.",
                "Correct. AA similarity: if two angles of one triangle are congruent to two of another, the third pair matches automatically by the angle sum, so all three angles agree and the triangles are similar with proportional sides.",
                "Two angles is entirely sufficient for similarity. Because the angles of a triangle sum to 180°, knowing two determines the third at no extra cost.",
                "This is true as a statement about congruence but it is not the strongest conclusion available. Similarity already follows from the two angles alone, with no side needed."
              ],
              "hint": "Distinguish what two angles fix — shape or size — and name the relationship that captures only that."
            },
            {
              "id": "cgs-017",
              "topic": "Similarity Ratios",
              "difficulty": "hard",
              "stem": "△ABC ~ △DEF with AB = 6, DE = 9, and BC = 8. Find EF.",
              "choices": [
                "5.33",
                "11",
                "12",
                "10.5"
              ],
              "correctIndex": 2,
              "explanations": [
                "5.33 is 8 × (6/9), scaling in the wrong direction. Since DE is larger than AB, △DEF is the bigger triangle and EF must exceed BC.",
                "11 adds the difference, 8 + 3, instead of multiplying by the ratio. Similarity scales lengths multiplicatively, not by adding a constant.",
                "Correct. The scale factor from ABC to DEF is DE/AB = 9/6 = 1.5. Corresponding sides scale by that factor, so EF = BC × 1.5 = 8 × 1.5 = 12.",
                "10.5 comes from an inconsistent ratio. Verify by checking that EF/BC equals DE/AB: 12/8 = 1.5 = 9/6. ✓"
              ],
              "hint": "Establish the scale factor from the pair of sides you know completely, then check its direction before applying it."
            },
            {
              "id": "cgs-018",
              "topic": "Reasoning: Choose the Criterion",
              "difficulty": "hard",
              "stem": "Two triangles share two pairs of congruent sides and one pair of congruent angles that is NOT between those sides. Why is congruence not guaranteed?",
              "choices": [
                "This is SSA, which can describe two genuinely different triangles — the ambiguous case.",
                "It is SAS, which is valid, so congruence IS guaranteed.",
                "Two sides and an angle are never enough information.",
                "It is only invalid when the triangles are obtuse."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. With the angle outside the two sides, the third side can often be placed two different ways, producing two non-congruent triangles. That is why SSA is not a criterion.",
                "SAS requires the angle to be INCLUDED between the two sides. Here it is not, which is the whole difference.",
                "SAS is two sides and an angle and is perfectly valid — the position of the angle is what matters.",
                "The ambiguity arises from the configuration, not from whether the triangle is obtuse."
              ],
              "hint": "Where the angle sits relative to the two sides is the entire question.",
              "kind": "reasoning"
            },
            {
              "id": "cgs-019",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "Two similar triangles have a scale factor of 4. A student says the perimeter of the larger is 16 times the smaller. What is the correct factor?",
              "choices": [
                "16, since perimeter scales with the square.",
                "4 — perimeter is a sum of LENGTHS, so it scales by the linear factor.",
                "64, matching the cube.",
                "The perimeter is unchanged."
              ],
              "correctIndex": 1,
              "explanations": [
                "16 = 4² is how AREA scales. Perimeter is not an area.",
                "Correct. Perimeter is the sum of side lengths, and every length scales by 4 — so the sum does too.",
                "64 = 4³ is how volume scales, and these are two-dimensional figures.",
                "Similar figures with a scale factor other than 1 have different perimeters."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "trig-laws",
          "title": "Laws of Sines & Cosines",
          "blurb": "Choosing between the two laws, solving ASA/AAS/SSS/SAS triangles, the trigonometric area formula, and the ambiguous case of SSA.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "laws-of-sines-cosines"
          ],
          "questions": [
            {
              "id": "tls-001",
              "topic": "Law of Sines",
              "difficulty": "medium",
              "stem": "In △ABC, m∠A = 30°, side a = 6, and m∠B = 45°. Find side b.",
              "choices": [
                "6√2",
                "3√2",
                "12",
                "6√3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The Law of Sines says a/sin A = b/sin B. So 6/sin 30° = b/sin 45°, giving 6/0.5 = 12 as the common ratio, and b = 12 · sin 45° = 12(√2/2) = 6√2 ≈ 8.49.",
                "3√2 halves the correct answer — it uses 6 as the common ratio rather than 12. Dividing 6 by sin 30° = 0.5 doubles it.",
                "12 is the common ratio a/sin A itself, not a side. It still has to be multiplied by sin B.",
                "6√3 would follow from using a 60° angle. sin 45° = √2/2, which carries the √2."
              ],
              "hint": "Compute a/sin A first — that single number is the same for all three sides. Then multiply it by sin B."
            },
            {
              "id": "tls-002",
              "topic": "Law of Cosines",
              "difficulty": "medium",
              "stem": "A triangle has sides of 5 and 8 with an included angle of 60°. Find the third side.",
              "choices": [
                "7",
                "√89",
                "13",
                "√129"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The Law of Cosines gives c² = a² + b² − 2ab·cos C = 25 + 64 − 2(5)(8)cos 60° = 89 − 80(0.5) = 89 − 40 = 49, so c = 7.",
                "√89 drops the −2ab·cos C term entirely, which is the same as assuming C = 90°. At 60° the correction is a real −40.",
                "13 adds the two sides. The third side of a triangle is always strictly less than that.",
                "√129 ADDS 40 instead of subtracting it, which is what happens if the sign of the cosine term is flipped. An angle under 90° makes the third side shorter than the right-angle case, not longer."
              ],
              "hint": "Law of Cosines is Pythagoras plus a correction term. Work out whether a 60° angle makes the opposite side longer or shorter than the right-angle case.",
              "diagram": "<svg viewBox=\"0 0 240 142\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"30,110 200,110 96,32\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><text x=\"58\" y=\"74\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5</text><text x=\"158\" y=\"66\" fill=\"#1f2024\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">8</text><text x=\"108\" y=\"130\" fill=\"#c0392b\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">c = ?</text><text x=\"96\" y=\"56\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">60°</text></svg>",
              "diagramCaption": "The 60° angle sits between the two known sides. Not drawn to scale"
            },
            {
              "id": "tls-003",
              "topic": "Choosing a Law",
              "difficulty": "medium",
              "stem": "You know all three side lengths of a triangle and want an angle. Which law applies?",
              "choices": [
                "Law of Cosines",
                "Law of Sines",
                "Either works equally well",
                "Neither — three sides do not determine the angles"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The Law of Cosines relates three sides and one angle, so with SSS you can rearrange it to cos C = (a² + b² − c²)/(2ab) and solve for any angle directly.",
                "The Law of Sines pairs each side with the angle OPPOSITE it, so it needs at least one angle already known. With SSS you have none, and the equation has two unknowns.",
                "They are not interchangeable. Which one applies is decided by what you have: SSS and SAS need the Law of Cosines; ASA, AAS and SSA need the Law of Sines.",
                "Three sides absolutely determine the angles — that is exactly what SSS congruence means."
              ],
              "hint": "One law connects three sides and one angle; the other connects two sides and two opposite angles. Which fits SSS?"
            },
            {
              "id": "tls-004",
              "topic": "The Ambiguous Case",
              "difficulty": "hard",
              "stem": "Given a = 8, b = 10 and m∠A = 40°, how many triangles are possible?",
              "choices": [
                "Exactly one",
                "Two",
                "None",
                "Infinitely many"
              ],
              "correctIndex": 1,
              "explanations": [
                "Exactly one would need the configuration to be unambiguous. This is SSA — two sides and a non-included angle — and the side opposite the known angle is the SHORTER one, which is the setup that can close two ways.",
                "Correct. Two. Check the height from C to line AB: h = b·sin A = 10·sin 40° ≈ 6.43. Since h < a < b (6.43 < 8 < 10), side a is long enough to reach the base but short enough to reach it twice — once with an acute angle B and once with its obtuse supplement. This is the ambiguous case of the Law of Sines, the trigonometric face of why SSA is not a congruence criterion.",
                "None would require a < h, meaning side a is too short to reach the base at all. Here 8 > 6.43, so it reaches.",
                "Infinitely many would need an angle-only condition, like AAA. Fixing two lengths pins the triangle down to finitely many options."
              ],
              "hint": "Compute h = b·sin A, the shortest distance from C to the base line, then compare it with a and with b. Three comparisons, three outcomes."
            },
            {
              "id": "tls-005",
              "topic": "Area with Trigonometry",
              "difficulty": "medium",
              "stem": "A triangle has sides of 7 and 10 with an included angle of 30°. What is its area?",
              "choices": [
                "17.5",
                "35",
                "70",
                "8.75"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Area = ½ab·sin C = ½(7)(10)sin 30° = 35(0.5) = 17.5.",
                "35 is ½ab without the sine factor, which would be the area only if C were 90°.",
                "70 is the full product ab. That is twice the area of the RECTANGLE on those sides, let alone the triangle.",
                "8.75 halves the answer once too often — perhaps by applying both the ½ and a further ½ from sin 30° twice over."
              ],
              "hint": "Same formula as the parallelogram area, halved: a triangle is half the parallelogram on the same two sides."
            },
            {
              "id": "tls-006",
              "topic": "Law of Sines",
              "difficulty": "medium",
              "stem": "In △ABC, m∠A = 50°, m∠B = 60° and side a = 9. Which setup finds side c?",
              "choices": [
                "c = 9·sin 70° / sin 50°",
                "c = 9·sin 50° / sin 70°",
                "c = 9·sin 60° / sin 50°",
                "c² = 9² + b² − 2(9)(b)cos 70°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. First find ∠C = 180° − 50° − 60° = 70°. Then the Law of Sines gives c/sin C = a/sin A, so c = a·sin C/sin A = 9·sin 70°/sin 50°.",
                "This inverts the ratio, dividing by the sine of the angle opposite the side you WANT rather than the one you know. It would make c smaller than a, but ∠C > ∠A so c must be larger.",
                "sin 60° pairs with side b, not side c. Each side goes with the angle directly opposite it.",
                "The Law of Cosines is not wrong here, but it introduces b, which is unknown. Using it would mean solving for b first — extra work when the Law of Sines answers directly."
              ],
              "hint": "Find the third angle first. Then pair each side with the angle across from it."
            },
            {
              "id": "tls-007",
              "topic": "Law of Cosines",
              "difficulty": "hard",
              "stem": "A triangle has sides 6, 7 and 9. Find the cosine of the angle opposite the side of length 9.",
              "choices": [
                "1/21",
                "2/21",
                "11/21",
                "−1/21"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Rearranged, cos C = (a² + b² − c²)/(2ab) with c = 9 opposite C: (36 + 49 − 81)/(2·6·7) = 4/84 = 1/21. Positive but tiny, so C is just under 90° — consistent with 6² + 7² = 85 being only slightly more than 9² = 81.",
                "2/21 would come from 4/42, halving the denominator. 2ab = 2(6)(7) = 84, not 42.",
                "11/21 comes from (36 + 49 + 81 − ...) or a sign slip in the numerator. The side opposite the angle is SUBTRACTED.",
                "−1/21 flips the sign, which would make C obtuse. Since 6² + 7² > 9², the angle is acute and the cosine is positive."
              ],
              "hint": "Rearrange c² = a² + b² − 2ab·cos C for cos C, and be careful which side is opposite the angle you want."
            },
            {
              "id": "tls-008",
              "topic": "Choosing a Law",
              "difficulty": "easy",
              "stem": "You know two angles and the side between them (ASA). Which law finds another side most directly?",
              "choices": [
                "Law of Sines",
                "Law of Cosines",
                "The Pythagorean theorem",
                "Neither applies to ASA"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Two angles give the third for free, so every angle is known along with one side — exactly the pairing the Law of Sines needs.",
                "The Law of Cosines needs at least two sides. Here only one is known, so it cannot be applied directly.",
                "The Pythagorean theorem applies only to right triangles, and nothing in ASA guarantees one.",
                "ASA is a standard solvable case. It is one of the configurations the Law of Sines exists for."
              ],
              "hint": "Count what you have: how many angles, how many sides? Then match that to what each law consumes."
            },
            {
              "id": "tls-009",
              "topic": "The Ambiguous Case",
              "difficulty": "hard",
              "stem": "Given a = 5, b = 12 and m∠A = 40°, how many triangles are possible?",
              "choices": [
                "Two",
                "Exactly one",
                "None",
                "Three"
              ],
              "correctIndex": 2,
              "explanations": [
                "Two requires h < a < b. Here a = 5 is smaller than h ≈ 7.71, so it never reaches the base even once.",
                "Exactly one would need a ≥ b, letting the side swing past the foot of the perpendicular. Here a is much smaller than b.",
                "Correct. None. The height from C to line AB is h = b·sin A = 12·sin 40° ≈ 7.71. Side a = 5 is shorter than that, so it cannot reach the base line at all and no triangle closes.",
                "Three is never an outcome of the ambiguous case. SSA yields zero, one, or two triangles."
              ],
              "hint": "Compute h = b·sin A and compare it to a. If the side is shorter than the perpendicular distance, it cannot reach."
            },
            {
              "id": "tls-010",
              "topic": "Applications",
              "difficulty": "medium",
              "stem": "Two roads leave a junction at an angle of 70°. One car drives 4 km along the first and another drives 6 km along the second. How far apart are they? (cos 70° ≈ 0.342)",
              "choices": [
                "√(52 − 48(0.342))",
                "√(52 + 48(0.342))",
                "10 km",
                "2 km"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The two roads and the line between the cars form a triangle with the 70° angle included between the 4 km and 6 km sides. The Law of Cosines gives d² = 4² + 6² − 2(4)(6)cos 70° = 16 + 36 − 48(0.342) = 52 − 16.4 ≈ 35.6, so d ≈ 5.97 km.",
                "Adding the cosine term instead of subtracting it would make the cars further apart than a right-angle turn would put them, which is backwards — an angle under 90° brings them closer.",
                "10 km is 4 + 6, the distance if the cars drove in exactly opposite directions (a 180° angle). At 70° they are much closer.",
                "2 km is 6 − 4, the distance if they drove along the SAME road in the same direction. That is the 0° case."
              ],
              "hint": "The junction angle sits between the two distances travelled. That is the included-angle setup."
            },
            {
              "id": "tls-011",
              "topic": "Law of Sines",
              "difficulty": "medium",
              "stem": "In a triangle, the largest angle is always opposite which side?",
              "choices": [
                "The longest side",
                "The shortest side",
                "The side of average length",
                "It depends on the triangle"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The Law of Sines makes each side proportional to the sine of its opposite angle, so bigger angles sit across from longer sides. It is also visible directly: opening an angle wider pushes the opposite side further apart.",
                "The shortest side sits opposite the SMALLEST angle — the same rule, read the other way.",
                "The average-length side sits opposite the middle angle, not the largest.",
                "It does not depend on the triangle. The side-angle ordering holds in every triangle without exception."
              ],
              "hint": "Picture opening one angle of a triangle wider while keeping the two sides fixed. What happens to the side across from it?"
            },
            {
              "id": "tls-012",
              "topic": "Law of Cosines",
              "difficulty": "easy",
              "stem": "For which angle C does the Law of Cosines c² = a² + b² − 2ab·cos C reduce to the Pythagorean theorem?",
              "choices": [
                "90°",
                "0°",
                "60°",
                "180°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. cos 90° = 0, so the −2ab·cos C term vanishes and the formula becomes c² = a² + b². The Pythagorean theorem is the right-angle special case of the Law of Cosines.",
                "At 0° the cosine is 1, giving c² = a² + b² − 2ab = (a − b)², so c = |a − b| — the degenerate case where the two sides lie along each other.",
                "At 60° the cosine is ½, leaving a correction of −ab. That is a real term, not zero.",
                "At 180° the cosine is −1, giving c² = (a + b)² — the other degenerate case, where the sides point in opposite directions."
              ],
              "hint": "The extra term disappears when the cosine is zero. At what angle is that?"
            },
            {
              "id": "tls-013",
              "topic": "Choosing a Law",
              "difficulty": "medium",
              "stem": "You are given two sides of a triangle and the angle between them (SAS), and asked for the third side. Which law applies?",
              "choices": [
                "Law of Sines",
                "Law of Cosines",
                "Either works equally well",
                "Neither — use the Pythagorean Theorem"
              ],
              "correctIndex": 1,
              "explanations": [
                "The Law of Sines needs an angle PAIRED WITH ITS OPPOSITE SIDE to form a usable ratio. In SAS the known angle sits between the known sides, so its opposite side is exactly the unknown — there is no complete ratio to start from.",
                "Correct. The Law of Cosines, c² = a² + b² − 2ab·cos C, is built for SAS: the included angle and its two adjacent sides are precisely its inputs, and it returns the opposite side directly.",
                "They are not interchangeable. Each law requires a specific pattern of given information, and SAS supplies what the Law of Cosines needs but not what the Law of Sines needs.",
                "The Pythagorean Theorem applies only to RIGHT triangles. Using it on a general triangle is described in the source text as the single most common error in trigonometry — the Law of Cosines is Pythagoras plus a correction term for the non-right angle."
              ],
              "hint": "Ask which law can start with the given data. One needs an angle-side pair that SAS does not provide."
            },
            {
              "id": "tls-014",
              "topic": "Law of Cosines and Pythagoras",
              "difficulty": "hard",
              "stem": "In the Law of Cosines c² = a² + b² − 2ab·cos C, what happens when C = 90°?",
              "choices": [
                "The formula becomes undefined.",
                "It reduces to the Pythagorean Theorem.",
                "It reduces to the Law of Sines.",
                "The correction term doubles."
              ],
              "correctIndex": 1,
              "explanations": [
                "Nothing becomes undefined. Cosine is perfectly well defined at 90°; it simply equals zero there.",
                "Correct. cos 90° = 0, so the term −2ab·cos C vanishes and the formula collapses to c² = a² + b². This is why the Law of Cosines is best understood as Pythagoras with a correction term measuring how far angle C departs from a right angle.",
                "The Law of Sines is a different relationship entirely, built on ratios of sines to opposite sides. No substitution turns one law into the other.",
                "The correction term disappears rather than doubling. It is largest when C is far from 90° and shrinks to nothing as C approaches it."
              ],
              "hint": "Evaluate the cosine at 90° and see what happens to the last term."
            },
            {
              "id": "tls-015",
              "topic": "The Ambiguous Case",
              "difficulty": "hard",
              "stem": "Why is SSA called the ambiguous case when using the Law of Sines?",
              "choices": [
                "The Law of Sines cannot be applied to SSA at all.",
                "The given data may correspond to two different triangles.",
                "The answer is always a non-real number.",
                "The triangle is always obtuse."
              ],
              "correctIndex": 1,
              "explanations": [
                "The Law of Sines can be applied to SSA — an angle is paired with its opposite side, which is exactly what it needs. The difficulty is with interpreting the result, not with starting.",
                "Correct. With two sides and a non-included angle, the side opposite the known angle can sometimes reach the base line in two places, producing two valid triangles — one acute and one obtuse at that vertex. This is the same ambiguity that disqualifies SSA as a congruence criterion.",
                "The computation returns an ordinary real sine value. The problem is that arcsine returns only one of the two angles that share that sine, and the supplementary angle may also be valid.",
                "Neither outcome is guaranteed. The ambiguity is precisely that BOTH an acute and an obtuse solution can exist for the same data — sometimes one, sometimes two, sometimes none."
              ],
              "hint": "Connect this back to why SSA fails as a congruence criterion. The same geometric freedom causes both problems."
            },
            {
              "id": "tls-016",
              "topic": "Trigonometric Area",
              "difficulty": "medium",
              "stem": "A triangle has sides a = 6 and b = 10 with an included angle C = 30°. Find its area.",
              "choices": [
                "15",
                "30",
                "60",
                "7.5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The trigonometric area formula is A = ½ab·sin C. Here A = ½(6)(10)(sin 30°) = ½(60)(0.5) = 15.",
                "30 is ½ab without the sine factor, which would be the area only if C were 90° (since sin 90° = 1). The angle here is 30°, so the sine factor halves it again.",
                "60 is the raw product ab, with neither the ½ nor the sine applied.",
                "7.5 applies the ½ twice, or uses sin 30° = 0.25. The sine of 30° is exactly 0.5."
              ],
              "hint": "The formula needs the angle BETWEEN the two sides. Recall the exact value of sin 30° rather than approximating."
            },
            {
              "id": "tls-017",
              "topic": "Law of Sines Setup",
              "difficulty": "medium",
              "stem": "In a triangle, angle A = 40°, angle B = 60°, and side a = 8. Which equation correctly finds side b?",
              "choices": [
                "b = 8 · sin 40° / sin 60°",
                "b = 8 · sin 60° / sin 40°",
                "b² = 8² + b² − 2(8)(b)cos 60°",
                "b = 8 · cos 60° / cos 40°"
              ],
              "correctIndex": 1,
              "explanations": [
                "This inverts the ratio. Since angle B (60°) is larger than angle A (40°), side b must be LONGER than side a — but this expression gives a value smaller than 8.",
                "Correct. The Law of Sines gives sin A / a = sin B / b, so b = a · sin B / sin A = 8 · sin 60° / sin 40°. Because B > A, the result exceeds 8, as the larger-angle-opposite-larger-side rule requires.",
                "This is a malformed Law of Cosines with b appearing on both sides. In any case AAS data is exactly what the Law of Sines handles directly.",
                "The Law of Sines uses sines, not cosines. Substituting cosine changes the relationship entirely and will not produce a correct length."
              ],
              "hint": "Set up the proportion first, then sanity-check the answer against which angle is larger."
            },
            {
              "id": "tls-018",
              "topic": "Reasoning: Choose the Law",
              "difficulty": "hard",
              "stem": "You know all three sides of a triangle and need an angle. Which law applies, and why not the other?",
              "choices": [
                "Law of Sines, because it relates sides and angles.",
                "Law of Cosines, because it is the only one that can start from three sides (SSS).",
                "Either works equally well.",
                "Neither — three sides do not determine the angles."
              ],
              "correctIndex": 1,
              "explanations": [
                "The Law of Sines needs an angle to already be known, since every one of its ratios pairs a side with its opposite ANGLE. With only sides, there is nothing to start from.",
                "Correct. Rearranged, the Law of Cosines gives cos C = (a² + b² − c²)/(2ab), which needs only the three sides. Once one angle is known, the Law of Sines can finish the job.",
                "The Law of Sines cannot begin from SSS at all, so they are not interchangeable here.",
                "SSS determines a triangle completely — that is exactly why SSS is a congruence criterion."
              ],
              "hint": "Write out what each law needs before it can be used. Which one can start with no angle at all?",
              "kind": "reasoning"
            },
            {
              "id": "tls-019",
              "topic": "Reasoning: The Ambiguous Case",
              "difficulty": "hard",
              "stem": "Using the Law of Sines with SSA data, a student finds sin B = 0.6 and reports B = 36.87° as the only answer. What has been overlooked?",
              "choices": [
                "Nothing; sine gives a unique angle.",
                "The obtuse solution: sin(180° − 36.87°) = 0.6 as well, so B ≈ 143.13° may also produce a valid triangle.",
                "The angle should have been found with cosine instead.",
                "Sine values above 0.5 have no solution."
              ],
              "correctIndex": 1,
              "explanations": [
                "Sine is positive in both the first and second quadrants, so an equation like sin B = 0.6 has two solutions between 0° and 180°.",
                "Correct. This is the ambiguous case of SSA. Both 36.87° and 143.13° have a sine of 0.6, and each must be tested to see whether the angles still sum to under 180°.",
                "Cosine would avoid the ambiguity, but the Law of Sines was the right tool for the data — the fix is to check the second solution, not change law.",
                "Sine takes every value from 0 to 1 for angles in this range."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "area-perimeter-measurement",
          "title": "Area, Perimeter & Measurement",
          "blurb": "Heron's formula, the apothem and regular-polygon area, rhombus and kite diagonals, sectors and arc length, geometric-mean segments, and composite figures.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "area-perimeter",
            "measurement"
          ],
          "questions": [
            {
              "id": "apm-001",
              "topic": "Heron's Formula",
              "difficulty": "hard",
              "stem": "A triangle has side lengths 5, 6 and 7. What is its area, using Heron's formula?",
              "choices": [
                "6√6",
                "15",
                "216",
                "3√21"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The semi-perimeter is s = (5 + 6 + 7)/2 = 9. Then A = √(s(s−a)(s−b)(s−c)) = √(9 · 4 · 3 · 2) = √216 = 6√6 ≈ 14.7.",
                "15 is close to the true value but is not it. Heron's product here is 216, and √216 is irrational — a whole-number area would be a coincidence.",
                "216 is the value under the radical, s(s−a)(s−b)(s−c). The square root is the final step.",
                "3√21 = √189, which would come from a slip in one of the four factors. Check s − a, s − b and s − c: they are 4, 3 and 2."
              ],
              "hint": "Half the perimeter first, then subtract each side from it in turn. Four numbers multiply under the root."
            },
            {
              "id": "apm-002",
              "topic": "Regular Polygons",
              "difficulty": "medium",
              "stem": "A regular hexagon has a side length of 10 units and an apothem of 5√3 units. What is its area?",
              "choices": [
                "150√3 sq units",
                "50√3 sq units",
                "75√3 sq units",
                "300√3 sq units"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A regular polygon's area is ½ × apothem × perimeter. The perimeter is 6 × 10 = 60, so A = ½(5√3)(60) = 150√3 ≈ 259.8 sq units.",
                "50√3 is the area of just ONE of the six triangular segments the hexagon splits into: ½(10)(5√3) = 25√3 — and even that is half of 50√3. Either way it is one piece, not the whole.",
                "75√3 halves the correct answer, which happens if the ½ is applied twice or the perimeter is taken as 30.",
                "300√3 is apothem × perimeter without the ½. That counts each triangular segment as a parallelogram."
              ],
              "hint": "Cut the hexagon into six triangles from the centre. The apothem is each one's height, and their bases add up to the perimeter.",
              "diagram": "<svg viewBox=\"0 0 240 164\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"120,18 178,51 178,117 120,150 62,117 62,51\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"120\" y1=\"84\" x2=\"178\" y2=\"51\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"84\" x2=\"178\" y2=\"117\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"84\" x2=\"120\" y2=\"150\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"84\" x2=\"62\" y2=\"117\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"84\" x2=\"62\" y2=\"51\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"84\" x2=\"120\" y2=\"18\" stroke=\"#6b6b76\" stroke-width=\"1.2\" stroke-linecap=\"round\"/><line x1=\"120\" y1=\"84\" x2=\"149\" y2=\"100\" stroke=\"#1f2024\" stroke-width=\"1.8\" stroke-linecap=\"round\"/><path d=\"M 144,92 L 150,102 L 140,108\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"126\" y=\"96\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">5√3</text><text x=\"186\" y=\"92\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">10</text></svg>",
              "diagramCaption": "Apothem to a side midpoint. Not drawn to scale"
            },
            {
              "id": "apm-003",
              "topic": "Quadrilateral Area",
              "difficulty": "medium",
              "stem": "A rhombus has an area of 84 sq units and one diagonal of length 12 units. What is the length of the other diagonal?",
              "choices": [
                "14 units",
                "7 units",
                "28 units",
                "21 units"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A rhombus has area ½d₁d₂, so 84 = ½(12)d₂ = 6d₂, giving d₂ = 14 units.",
                "7 units drops the ½ from the area formula, solving 84 = 12d₂ instead. This is the most common slip on this question — it treats the rhombus as a rectangle on its diagonals.",
                "28 units doubles the correct answer, applying the ½ on the wrong side of the equation.",
                "21 units would follow from 84 = 4d₂ or a similar arithmetic slip. Half of 12 is 6, and 84 ÷ 6 = 14."
              ],
              "hint": "Write the area formula first, substitute both knowns, and check whether the ½ is helping you or being dropped."
            },
            {
              "id": "apm-004",
              "topic": "Quadrilateral Area",
              "difficulty": "easy",
              "stem": "A kite has diagonals measuring 16 inches and 10 inches. What is its area?",
              "choices": [
                "80 sq inches",
                "160 sq inches",
                "100 sq inches",
                "40 sq inches"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A kite's diagonals are perpendicular, so the same formula as the rhombus applies: A = ½d₁d₂ = ½(16)(10) = 80 sq inches.",
                "160 is the full product of the diagonals, which is the area of the RECTANGLE they span — exactly twice the kite.",
                "100 is 10², using only one diagonal. Both are needed.",
                "40 quarters the product rather than halving it."
              ],
              "hint": "A kite's diagonals meet at right angles, so it obeys the same area rule as a rhombus."
            },
            {
              "id": "apm-005",
              "topic": "Circles",
              "difficulty": "medium",
              "stem": "What is the area of a 90° sector of a circle with radius 8 units? (Leave in terms of π.)",
              "choices": [
                "16π sq units",
                "64π sq units",
                "8π sq units",
                "2π sq units"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A sector is the fraction of the circle its angle cuts: (90/360)πr² = ¼ · π(64) = 16π sq units.",
                "64π is the area of the WHOLE circle. The quarter fraction still has to be applied.",
                "8π is the ARC LENGTH plus a slip, or half the correct sector. Arc length uses circumference; area uses πr².",
                "2π is the arc length of this sector, (90/360)(2π·8) = 4π — and even that is not 2π. Check which formula produces square units."
              ],
              "hint": "90° is a quarter turn. Take that fraction of the whole circle's area."
            },
            {
              "id": "apm-006",
              "topic": "Circles",
              "difficulty": "medium",
              "stem": "What is the arc length of a 90° arc in a circle with radius 8 units? (Leave in terms of π.)",
              "choices": [
                "4π units",
                "16π units",
                "2π units",
                "8π units"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Arc length is the same fraction of the CIRCUMFERENCE: (90/360)(2πr) = ¼(16π) = 4π units.",
                "16π is the whole circumference, 2π(8). The quarter fraction has still to be applied.",
                "2π halves the correct answer — perhaps by using πr instead of 2πr for the circumference.",
                "8π is half the circumference, which would be a 180° arc."
              ],
              "hint": "Arc length is to circumference what sector area is to area. Which of the two do you need here — and note the units are linear."
            },
            {
              "id": "apm-007",
              "topic": "Quadrilateral Measurement",
              "difficulty": "medium",
              "stem": "A rhombus has diagonals of length 6 and 8. What is the length of one of its sides?",
              "choices": [
                "5 units",
                "25 units",
                "7 units",
                "10 units"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The diagonals of a rhombus are perpendicular bisectors of each other, so they cut it into four congruent right triangles with legs 3 and 4 — half of each diagonal. The side is the hypotenuse: √(9 + 16) = 5 units.",
                "25 is the square of the side, 3² + 4². The root is the last step.",
                "7 adds the half-diagonals, 3 + 4. They are perpendicular legs, so they combine by Pythagoras.",
                "10 is the average of the diagonals, or half their sum. The side is shorter than either full diagonal."
              ],
              "hint": "The diagonals bisect each other at right angles. Look at just one of the four right triangles they create.",
              "diagram": "<svg viewBox=\"0 0 240 168\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"120,20 190,80 120,140 50,80\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"120\" y1=\"20\" x2=\"120\" y2=\"140\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><line x1=\"50\" y1=\"80\" x2=\"190\" y2=\"80\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><path d=\"M 120,68 L 132,68 L 132,80\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"126\" y=\"52\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3</text><text x=\"152\" y=\"74\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4</text><text x=\"160\" y=\"44\" fill=\"#c0392b\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><text x=\"84\" y=\"158\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">diagonals 6 and 8</text></svg>",
              "diagramCaption": "Four congruent right triangles. Not drawn to scale"
            },
            {
              "id": "apm-008",
              "topic": "Regular Polygons",
              "difficulty": "easy",
              "stem": "What is the apothem of a regular polygon?",
              "choices": [
                "The perpendicular distance from the centre to the midpoint of any side",
                "The total length around the polygon boundary",
                "The distance from the centre to any vertex",
                "The length of one side of the polygon"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The apothem runs from the centre perpendicular to a side, landing on that side's midpoint. It is also the radius of the inscribed circle, which is why it appears as the height in the ½ × apothem × perimeter area formula.",
                "That is the perimeter.",
                "That is the circumradius — the radius of the CIRCUMSCRIBED circle, which reaches the corners. The apothem is shorter, reaching only to the edges.",
                "The side length is a separate measurement. In a regular hexagon of side 10 the apothem is 5√3 ≈ 8.66, not 10."
              ],
              "hint": "Two radii can be drawn in a regular polygon: one to a corner and one to an edge. This is the shorter of the two."
            },
            {
              "id": "apm-009",
              "topic": "Right Triangle Measurement",
              "difficulty": "medium",
              "stem": "In a right triangle, an altitude drawn to the hypotenuse divides it into segments of 2 and 8 units. What is the length of the altitude?",
              "choices": [
                "4 units",
                "16 units",
                "5 units",
                "6 units"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The altitude to the hypotenuse is the geometric mean of the two segments it creates: h = √(2 × 8) = √16 = 4 units.",
                "16 is h², one square root short.",
                "5 is the arithmetic mean, (2 + 8)/2. The relationship here is geometric — multiply and take the root.",
                "6 would follow from segments of 4 and 9. Here they are 2 and 8, whose product is 16."
              ],
              "hint": "Multiply the two pieces of the hypotenuse and take the square root."
            },
            {
              "id": "apm-010",
              "topic": "Right Triangle Measurement",
              "difficulty": "hard",
              "stem": "In a right triangle, an altitude to the hypotenuse splits it into segments of 3 and 12 units. What is the length of the shorter leg?",
              "choices": [
                "3√5 units",
                "6 units",
                "9 units",
                "6√5 units"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Each leg is the geometric mean of the whole hypotenuse and the segment ADJACENT to it. The hypotenuse is 3 + 12 = 15, so the shorter leg (adjacent to the 3) is √(15 × 3) = √45 = 3√5 ≈ 6.7 units.",
                "6 is the ALTITUDE, √(3 × 12) = √36. That is the geometric mean of the two segments, not of the hypotenuse and a segment.",
                "9 does not follow from any of the three geometric-mean relationships in this figure.",
                "6√5 = √180 = √(15 × 12) is the LONGER leg, the one adjacent to the 12-unit segment."
              ],
              "hint": "Three geometric means live in this figure: one for the altitude and one for each leg. The leg ones use the full hypotenuse.",
              "diagram": "<svg viewBox=\"0 0 240 140\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"25,110 215,110 63,32\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><line x1=\"63\" y1=\"32\" x2=\"63\" y2=\"110\" stroke=\"#1f2024\" stroke-width=\"1.8\" stroke-linecap=\"round\"/><path d=\"M 63,98 L 75,98 L 75,110\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><path d=\"M 55,40 L 65,46 L 59,54\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.4\"/><text x=\"38\" y=\"128\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3</text><text x=\"134\" y=\"128\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">12</text><text x=\"30\" y=\"74\" fill=\"#c0392b\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text><text x=\"68\" y=\"76\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">h</text></svg>",
              "diagramCaption": "The altitude falls to the hypotenuse. Not drawn to scale"
            },
            {
              "id": "apm-011",
              "topic": "Measurement and Units",
              "difficulty": "medium",
              "stem": "A central angle of 90° in a circle of radius 6 subtends an arc. What is that arc's length in radians-based form, using s = rθ?",
              "choices": [
                "3π units",
                "6π units",
                "1.5π units",
                "270 units"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Converting first: 90° = π/2 radians. Then s = rθ = 6(π/2) = 3π units. The degree-based route agrees: (90/360)(2π·6) = ¼(12π) = 3π.",
                "6π is half the circumference, which would be a 180° arc.",
                "1.5π halves the correct answer, which happens if the radius is taken as 3 or the angle as 45°.",
                "270 multiplies 90 by 3 and keeps degrees. Arc length is a distance, so it cannot be measured in degrees — the units alone rule this out."
              ],
              "hint": "s = rθ only works when θ is in radians. Convert the 90° first, then multiply."
            },
            {
              "id": "apm-012",
              "topic": "Composite Area",
              "difficulty": "medium",
              "stem": "A running track is a rectangle 100 m by 60 m with a semicircle of diameter 60 m attached to each of the two shorter ends. What is the total enclosed area?",
              "choices": [
                "6000 + 900π m²",
                "6000 + 1800π m²",
                "6000 + 3600π m²",
                "6000 + 450π m²"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The rectangle contributes 100 × 60 = 6000 m². The two semicircles of diameter 60 (radius 30) together make one full circle of area π(30²) = 900π m². Total: 6000 + 900π ≈ 8827 m².",
                "1800π doubles the circle — it treats each semicircle as a whole circle rather than two halves making one.",
                "3600π uses the DIAMETER as the radius: π(60²). The radius is half the diameter, so 30.",
                "450π halves the circle's area, counting only one of the two semicircles."
              ],
              "hint": "Two semicircles of the same diameter make exactly one circle. And check whether 60 is the radius or the diameter.",
              "diagram": "<svg viewBox=\"0 0 240 132\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"60\" y1=\"40\" x2=\"180\" y2=\"40\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><line x1=\"60\" y1=\"110\" x2=\"180\" y2=\"110\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linecap=\"round\"/><path d=\"M 60,40 A 35 35 0 0 0 60,110\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><path d=\"M 180,40 A 35 35 0 0 1 180,110\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"60\" y1=\"40\" x2=\"60\" y2=\"110\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"180\" y1=\"40\" x2=\"180\" y2=\"110\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"104\" y=\"80\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">100 m</text><text x=\"186\" y=\"78\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">60 m</text></svg>",
              "diagramCaption": "A rectangle capped by two semicircles. Not drawn to scale"
            },
            {
              "id": "apm-013",
              "topic": "Heron's Formula",
              "difficulty": "hard",
              "stem": "Find the area of a triangle with sides 5, 6 and 7 using Heron's formula.",
              "choices": [
                "6√6",
                "9",
                "15",
                "18"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The semi-perimeter is s = (5 + 6 + 7)/2 = 9. Then A = √(s(s−a)(s−b)(s−c)) = √(9 · 4 · 3 · 2) = √216 = 6√6 ≈ 14.7.",
                "9 is the semi-perimeter s, an intermediate value rather than the area.",
                "15 comes from ½ · 5 · 6, treating two of the sides as base and height. That only works when those sides are perpendicular, which they are not here.",
                "18 is the full perimeter, 5 + 6 + 7. Perimeter and area are different measurements."
              ],
              "hint": "Compute the semi-perimeter first, then subtract each side from it in turn before multiplying."
            },
            {
              "id": "apm-014",
              "topic": "Trapezoid Area",
              "difficulty": "medium",
              "stem": "A trapezoid has area 48, height 6, and one base of length 10. Find the other base.",
              "choices": [
                "6",
                "8",
                "16",
                "4"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. From A = ½(b₁ + b₂)h we get 48 = ½(10 + b)(6) = 3(10 + b). So 16 = 10 + b and b = 6.",
                "8 is the average of the two bases, (10 + 6)/2. That average is an intermediate quantity, not the missing base.",
                "16 is the SUM of the two bases, b₁ + b₂. The sum still has to have the known base removed from it.",
                "4 comes from an arithmetic slip in isolating b. Work carefully: 48 divided by 3 gives 16, then subtract 10."
              ],
              "hint": "Substitute everything known into the area formula, then unwind it one operation at a time."
            },
            {
              "id": "apm-015",
              "topic": "Rhombus Area",
              "difficulty": "medium",
              "stem": "A rhombus has area 84 and one diagonal of length 12. Find the other diagonal.",
              "choices": [
                "7",
                "14",
                "12",
                "28"
              ],
              "correctIndex": 1,
              "explanations": [
                "7 is 84/12, which divides the area by the diagonal but skips the ½ in the formula. The ½ must be undone as well.",
                "Correct. For a rhombus (or kite), A = ½d₁d₂. So 84 = ½(12)d = 6d, giving d = 14.",
                "12 repeats the given diagonal. The two diagonals of a rhombus are generally different lengths — equal diagonals would make it a square.",
                "28 doubles the correct answer, applying the ½ in the wrong direction. Dividing 84 by 6 gives 14."
              ],
              "hint": "The diagonal formula carries a factor of ½. Account for it before dividing."
            },
            {
              "id": "apm-016",
              "topic": "Regular Polygon Area",
              "difficulty": "hard",
              "stem": "A regular hexagon has apothem 5√3 and perimeter 60. Find its area.",
              "choices": [
                "150√3",
                "300√3",
                "75√3",
                "60√3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The area of a regular polygon is A = ½ · apothem · perimeter = ½(5√3)(60) = 150√3 ≈ 259.8.",
                "300√3 is apothem × perimeter with the ½ omitted. That doubles the true area.",
                "75√3 halves the correct answer, applying the ½ twice.",
                "60√3 multiplies the apothem by 12 rather than by half the perimeter. Substitute the given perimeter of 60 directly into the formula."
              ],
              "hint": "The regular-polygon area formula mirrors the triangle formula: half of one length times another."
            },
            {
              "id": "apm-017",
              "topic": "Equilateral Triangle Area",
              "difficulty": "medium",
              "stem": "Find the area of an equilateral triangle with side 6. (Use A = (√3/4)s².)",
              "choices": [
                "9√3",
                "36√3",
                "18√3",
                "6√3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A = (√3/4)s² = (√3/4)(36) = 9√3 ≈ 15.6.",
                "36√3 substitutes s² = 36 but drops the /4. The denominator matters.",
                "18√3 divides by 2 instead of 4. The formula's denominator is 4, which comes from combining the ½ of the triangle area formula with the height (√3/2)s.",
                "6√3 uses s rather than s². The formula squares the side."
              ],
              "hint": "Square the side first, then apply the full coefficient including its denominator."
            },
            {
              "id": "apm-018",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "A rectangle's dimensions are doubled. A student says the area doubles. What actually happens, and why?",
              "choices": [
                "The area doubles, since both dimensions changed together.",
                "The area quadruples, because both length and width are doubled and area is their product.",
                "The area is 8 times bigger.",
                "The area stays the same."
              ],
              "correctIndex": 1,
              "explanations": [
                "Doubling ONE dimension doubles the area. Doubling both multiplies it twice over.",
                "Correct. Area = length × width, so doubling each gives (2l)(2w) = 4lw — four times the original.",
                "8× is the volume effect for a three-dimensional figure, where three dimensions each double.",
                "Both dimensions grew, so the area cannot be unchanged."
              ],
              "hint": "Write the new area as a product and see how many factors of 2 appear.",
              "kind": "reasoning"
            },
            {
              "id": "apm-019",
              "topic": "Reasoning: What's Missing",
              "difficulty": "medium",
              "stem": "A student is asked for the area of a trapezoid given only the two parallel sides, and says it is impossible. Are they right?",
              "choices": [
                "No — the two parallel sides are enough.",
                "Yes. The height is also needed, since the formula averages the parallel sides and multiplies by the perpendicular distance between them.",
                "Yes, but only because the angles are missing.",
                "No — the perimeter can be used instead."
              ],
              "correctIndex": 1,
              "explanations": [
                "Two trapezoids can share both parallel sides and have completely different areas if one is taller.",
                "Correct. A = ½(b₁ + b₂)h. Without h there is no way to pin the area down, because the trapezoid could be any height.",
                "The angles are one way to RECOVER the height if a slant side is known, but the missing ingredient is the height itself.",
                "Perimeter does not determine area — shapes with the same perimeter can enclose very different amounts."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "coordinate-conics",
          "title": "Circles, Parabolas & Constructions",
          "blurb": "Standard form and completing the square for circles, the focus-directrix definition of a parabola, and the compass-and-straightedge constructions behind both.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "circles-coordinate-plane",
            "parabolas-coordinate-plane",
            "constructions-circles"
          ],
          "questions": [
            {
              "id": "ccp-001",
              "topic": "Circles on the Coordinate Plane",
              "difficulty": "medium",
              "stem": "What are the centre and radius of the circle (x − 3)² + (y + 1)² = 25?",
              "choices": [
                "centre (3, −1), radius 5",
                "centre (−3, 1), radius 5",
                "centre (3, −1), radius 25",
                "centre (3, 1), radius 5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Standard form is (x − h)² + (y − k)² = r², so the centre is (h, k) = (3, −1) — note that (y + 1) is (y − (−1)) — and r = √25 = 5.",
                "This flips both signs. The form SUBTRACTS the coordinates, so (x − 3) gives h = +3, not −3.",
                "25 is r², not r. The square root is the last step.",
                "The y-coordinate sign is wrong. (y + 1) means k = −1, because +1 is what you get by subtracting −1."
              ],
              "hint": "Standard form subtracts. Rewrite (y + 1) as (y − ?) and read the centre straight off."
            },
            {
              "id": "ccp-002",
              "topic": "Circles on the Coordinate Plane",
              "difficulty": "hard",
              "stem": "Complete the square to find the radius of the circle x² + y² + 4x − 6y − 12 = 0.",
              "choices": [
                "5",
                "25",
                "12",
                "√12"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Group and complete: (x² + 4x + 4) + (y² − 6y + 9) = 12 + 4 + 9, giving (x + 2)² + (y − 3)² = 25. So r² = 25 and r = 5, centred at (−2, 3).",
                "25 is r². One square root short.",
                "12 is the constant that was moved across before the completing terms were added. Both 4 and 9 still have to be added to it.",
                "√12 uses the constant before completing the square. The 4 and the 9 added to the left must also be added to the right."
              ],
              "hint": "Half the x-coefficient squared, and half the y-coefficient squared. Add both to BOTH sides."
            },
            {
              "id": "ccp-003",
              "topic": "Circles on the Coordinate Plane",
              "difficulty": "medium",
              "stem": "Write the equation of the circle with centre (2, −5) and radius 3.",
              "choices": [
                "(x − 2)² + (y + 5)² = 9",
                "(x + 2)² + (y − 5)² = 9",
                "(x − 2)² + (y + 5)² = 3",
                "(x − 2)² + (y − 5)² = 9"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Substituting h = 2, k = −5, r = 3 into (x − h)² + (y − k)² = r² gives (x − 2)² + (y − (−5))² = 3², i.e. (x − 2)² + (y + 5)² = 9.",
                "Both signs are flipped. The form subtracts the centre's coordinates, so a centre of +2 produces (x − 2).",
                "The right-hand side must be r², which is 9, not r.",
                "The y term has the wrong sign. Subtracting −5 gives +5 inside the bracket."
              ],
              "hint": "Substitute into the standard form carefully, remembering that subtracting a negative flips the sign."
            },
            {
              "id": "ccp-004",
              "topic": "Circles on the Coordinate Plane",
              "difficulty": "medium",
              "stem": "A circle has a diameter with endpoints (1, 2) and (7, 10). What is its centre and radius?",
              "choices": [
                "centre (4, 6), radius 5",
                "centre (4, 6), radius 10",
                "centre (3, 4), radius 5",
                "centre (8, 12), radius 5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The centre is the midpoint of the diameter: ((1+7)/2, (2+10)/2) = (4, 6). The diameter's length is √(6² + 8²) = 10, so the radius is 5.",
                "10 is the DIAMETER, not the radius. Halve it.",
                "(3, 4) is the vector from one endpoint to the centre, or half the difference — not the midpoint itself.",
                "(8, 12) is the sum of the coordinates without halving."
              ],
              "hint": "The centre is the midpoint. The radius is half the distance between the endpoints — do not stop at the diameter."
            },
            {
              "id": "ccp-005",
              "topic": "Parabolas",
              "difficulty": "medium",
              "stem": "Geometrically, a parabola is the set of all points that are:",
              "choices": [
                "equidistant from a fixed point (the focus) and a fixed line (the directrix)",
                "equidistant from two fixed points",
                "a fixed distance from a single fixed point",
                "the sum of whose distances from two fixed points is constant"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. That is the defining property. Every point on a parabola is exactly as far from the focus as it is from the directrix, and the vertex is the point halfway between them.",
                "Points equidistant from two fixed points form the PERPENDICULAR BISECTOR of the segment joining them — a straight line, not a curve.",
                "A fixed distance from a single point describes a CIRCLE.",
                "A constant SUM of distances from two fixed points describes an ELLIPSE."
              ],
              "hint": "Each of the four descriptions defines a different figure. Which one involves a point and a line rather than two points?",
              "diagram": "<svg viewBox=\"0 0 240 152\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><path d=\"M 40,20 Q 120,150 200,20\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\"/><line x1=\"30\" y1=\"128\" x2=\"210\" y2=\"128\" stroke=\"#6b6b76\" stroke-width=\"1.6\" stroke-linecap=\"round\"/><circle cx=\"120\" cy=\"62\" r=\"3.2\" fill=\"#c0392b\"/><circle cx=\"120\" cy=\"95\" r=\"3.2\" fill=\"#1f2024\"/><circle cx=\"70\" cy=\"51\" r=\"3.2\" fill=\"#7c3aed\"/><line x1=\"70\" y1=\"51\" x2=\"120\" y2=\"62\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"70\" y1=\"51\" x2=\"70\" y2=\"128\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"126\" y=\"58\" fill=\"#c0392b\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">focus</text><text x=\"126\" y=\"100\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">vertex</text><text x=\"150\" y=\"142\" fill=\"#6b6b76\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">directrix</text><text x=\"40\" y=\"44\" fill=\"#1f2024\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">P</text></svg>",
              "diagramCaption": "The two dashed distances from P are always equal. Not drawn to scale"
            },
            {
              "id": "ccp-006",
              "topic": "Parabolas",
              "difficulty": "hard",
              "stem": "A parabola has its vertex at the origin and its focus at (0, 2). What is its equation?",
              "choices": [
                "x² = 8y",
                "y² = 8x",
                "x² = 2y",
                "x² = 4y"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. For a vertical parabola with vertex at the origin the form is x² = 4py, where p is the directed distance from vertex to focus. Here p = 2, so x² = 8y. The directrix is the line y = −2.",
                "y² = 8x opens sideways, which happens when the focus is on the x-axis. This focus is on the y-axis, so the parabola opens upward.",
                "x² = 2y would put the focus at (0, ½), since 4p = 2 gives p = ½.",
                "x² = 4y would put the focus at (0, 1). The 4p must equal 8 when p = 2."
              ],
              "hint": "Use x² = 4py and be careful: p is the vertex-to-focus distance, and it gets multiplied by 4."
            },
            {
              "id": "ccp-007",
              "topic": "Parabolas",
              "difficulty": "medium",
              "stem": "For the parabola x² = 12y, where is the directrix?",
              "choices": [
                "y = −3",
                "y = 3",
                "x = −3",
                "y = −12"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Comparing with x² = 4py gives 4p = 12, so p = 3. The focus is at (0, 3) and the directrix is the mirror line on the other side of the vertex: y = −3.",
                "y = 3 is the height of the FOCUS, not the directrix. They sit on opposite sides of the vertex.",
                "x = −3 is a vertical line, which would be the directrix of a sideways-opening parabola. This one opens upward.",
                "y = −12 uses 4p rather than p. Divide by 4 first."
              ],
              "hint": "Read 4p off the equation, halve it twice to get p, then place the directrix the same distance below the vertex as the focus is above."
            },
            {
              "id": "ccp-008",
              "topic": "Constructions",
              "difficulty": "medium",
              "stem": "To construct the perpendicular bisector of segment AB with compass and straightedge, what do you do first?",
              "choices": [
                "Draw arcs of equal radius (more than half of AB) centred at A and at B",
                "Measure AB with a ruler and mark the midpoint",
                "Draw a circle with AB as its diameter",
                "Draw arcs of any two different radii from A and B"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Equal radii from both endpoints make the two intersection points equidistant from A and B. Since the perpendicular bisector IS the set of points equidistant from A and B, the line through those two intersections is it. The radius must exceed half of AB or the arcs never meet.",
                "Measuring is not a construction. Compass-and-straightedge work is valid precisely because it uses no marked ruler — the point is to produce the midpoint, not to measure it.",
                "A circle on AB as diameter passes through A and B, and its centre is already the midpoint you are trying to find — so this assumes the answer.",
                "Different radii would give points that are NOT equidistant from A and B, so the line through them would not be the perpendicular bisector."
              ],
              "hint": "The perpendicular bisector is the locus of points equidistant from the two endpoints. What does the compass have to do to guarantee equal distances?",
              "diagram": "<svg viewBox=\"0 0 240 164\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"50\" y1=\"80\" x2=\"190\" y2=\"80\" stroke=\"#7c3aed\" stroke-width=\"2.2\" stroke-linecap=\"round\"/><circle cx=\"50\" cy=\"80\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"190\" cy=\"80\" r=\"3.2\" fill=\"#7c3aed\"/><circle cx=\"50\" cy=\"80\" r=\"90\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.2\"/><circle cx=\"190\" cy=\"80\" r=\"90\" fill=\"none\" stroke=\"#6b6b76\" stroke-width=\"1.2\"/><line x1=\"120\" y1=\"4\" x2=\"120\" y2=\"156\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"40\" y=\"96\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">A</text><text x=\"192\" y=\"96\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">B</text><text x=\"126\" y=\"20\" fill=\"#c0392b\" font-size=\"10.5\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">⟂ bisector</text></svg>",
              "diagramCaption": "Equal radii from both endpoints. Not drawn to scale"
            },
            {
              "id": "ccp-009",
              "topic": "Constructions",
              "difficulty": "medium",
              "stem": "To inscribe a circle inside a triangle, which construction locates the centre?",
              "choices": [
                "The intersection of the three angle bisectors",
                "The intersection of the three perpendicular bisectors",
                "The intersection of the three medians",
                "The intersection of the three altitudes"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The angle bisectors meet at the incentre, which is equidistant from all three SIDES — exactly what a circle tangent to all three sides needs. The radius is the perpendicular distance from that point to any side.",
                "Perpendicular bisectors meet at the circumcentre, which is equidistant from the three VERTICES. That gives the circle THROUGH the corners, not the one inside touching the edges.",
                "Medians meet at the centroid, the balance point. It has no equidistance property.",
                "Altitudes meet at the orthocentre, which has no equidistance property either and can fall outside the triangle."
              ],
              "hint": "An inscribed circle touches the sides. Which centre is equidistant from the sides rather than the corners?"
            },
            {
              "id": "ccp-010",
              "topic": "Constructions",
              "difficulty": "hard",
              "stem": "You are given a circle and a point P outside it, and must construct a tangent from P. Why is the circle with diameter OP (where O is the centre) the key step?",
              "choices": [
                "Because any angle inscribed in a semicircle is a right angle, so where it meets the original circle the radius is perpendicular to PT",
                "Because it bisects the segment OP",
                "Because it has the same radius as the original circle",
                "Because it passes through the centre O"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A tangent is perpendicular to the radius at the point of tangency, so the tangent point T must satisfy ∠OTP = 90°. Points seeing OP at a right angle lie exactly on the circle with OP as diameter — Thales' theorem — so intersecting that circle with the original one finds T.",
                "It does bisect OP, in the sense that its centre is OP's midpoint, but that is a by-product of the construction rather than the reason it works.",
                "It generally does not have the same radius, and the radius is not what makes the construction work.",
                "It passes through O, but so do infinitely many circles. What matters is the right angle the DIAMETER guarantees."
              ],
              "hint": "What angle must the radius make with the tangent at the point of contact? Then ask which locus of points sees a fixed segment at that angle."
            },
            {
              "id": "ccp-011",
              "topic": "Circles on the Coordinate Plane",
              "difficulty": "easy",
              "stem": "What is the measure of an angle inscribed in a semicircle?",
              "choices": [
                "90°",
                "60°",
                "180°",
                "It depends on the radius"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A semicircle is a 180° arc, and an inscribed angle is half its intercepted arc, so the angle is 90°. This is Thales' theorem, and it is what makes the tangent construction from an external point work.",
                "60° would come from a 120° arc, not a semicircle.",
                "180° is the ARC, not the inscribed angle standing on it. The angle is half the arc.",
                "It does not depend on the radius. Every angle inscribed in a semicircle is a right angle, whatever the circle's size."
              ],
              "hint": "Halve the arc. A semicircle is how many degrees?"
            },
            {
              "id": "ccp-012",
              "topic": "Coordinate Geometry History",
              "difficulty": "easy",
              "stem": "Who published 'La Géométrie' in 1637, introducing the Cartesian coordinate system that bridges algebra and geometry?",
              "choices": [
                "René Descartes",
                "Johann Lambert",
                "Euclid",
                "Leonhard Euler"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Descartes' La Géométrie fused algebra with geometry by putting figures on a coordinate grid — which is why the plane bears his name and why a circle can be written as an equation at all.",
                "Johann Lambert worked in the 18th century on non-Euclidean geometry and on proving π irrational.",
                "Euclid wrote the Elements around 300 BC, roughly two thousand years earlier and entirely without coordinates.",
                "Euler was an 18th-century mathematician — the Euler Line is his. La Géométrie predates him by a century."
              ],
              "hint": "The adjective for the coordinate plane is taken from this person's name."
            },
            {
              "id": "ccp-013",
              "topic": "Equation of a Circle",
              "difficulty": "easy",
              "stem": "Give the center and radius of the circle (x − 3)² + (y + 2)² = 25.",
              "choices": [
                "Center (3, −2), radius 5",
                "Center (−3, 2), radius 5",
                "Center (3, −2), radius 25",
                "Center (3, 2), radius 5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. In the form (x − h)² + (y − k)² = r², the center is (h, k) and the radius is √r². Here (y + 2) reads as (y − (−2)), so k = −2, and r = √25 = 5.",
                "This flips both signs. The signs in the equation are already reversed relative to the center coordinates, so reversing them again returns you to the wrong place.",
                "The center is right but 25 is r², not r. The square root step was skipped.",
                "The x-coordinate is right but the y-coordinate keeps the sign as written. Since the form subtracts k, a plus sign in the equation means a negative coordinate."
              ],
              "hint": "The standard form subtracts the center coordinates, so read the signs as reversed — and remember the right side is r squared."
            },
            {
              "id": "ccp-014",
              "topic": "Completing the Square",
              "difficulty": "hard",
              "stem": "Find the radius of the circle x² + y² − 6x + 8y + 9 = 0.",
              "choices": [
                "3",
                "4",
                "5",
                "9"
              ],
              "correctIndex": 1,
              "explanations": [
                "3 comes from taking √9 using the constant term as written. That constant has to be moved and adjusted by the completing-the-square step before it means anything.",
                "Correct. Group and complete the square: (x² − 6x) + (y² + 8y) = −9 becomes (x − 3)² − 9 + (y + 4)² − 16 = −9, so (x − 3)² + (y + 4)² = 16 and r = 4.",
                "5 would follow from r² = 25, which comes from adding 9 and 16 without accounting for the −9 already on the right side.",
                "9 is the original constant term, not the radius. It changes value during the completing-the-square process."
              ],
              "hint": "Complete the square on x and y separately, tracking carefully what each step adds to both sides."
            },
            {
              "id": "ccp-015",
              "topic": "Parabola Definition",
              "difficulty": "medium",
              "stem": "A parabola is defined as the set of all points equidistant from which two objects?",
              "choices": [
                "Two fixed points (the foci)",
                "A fixed point and a fixed line",
                "Two fixed lines",
                "A fixed point and a circle"
              ],
              "correctIndex": 1,
              "explanations": [
                "Equidistance from two fixed points gives the perpendicular bisector of the segment joining them — a straight line. A constant SUM of distances to two foci gives an ellipse, and a constant difference gives a hyperbola.",
                "Correct. A parabola is the locus of points equidistant from a fixed point (the focus) and a fixed line (the directrix). Every point on the curve has the property that its distance to the focus equals its perpendicular distance to the directrix.",
                "Points equidistant from two lines lie on an angle bisector, or on a line midway between them if the two are parallel. Neither is a parabola.",
                "Equidistance from a point and a circle produces an ellipse or hyperbola depending on the configuration, not a parabola."
              ],
              "hint": "One of the two defining objects is zero-dimensional and the other is one-dimensional. Name each."
            },
            {
              "id": "ccp-016",
              "topic": "Circle Through a Point",
              "difficulty": "medium",
              "stem": "A circle has center (2, −1) and passes through the point (5, 3). What is its equation?",
              "choices": [
                "(x − 2)² + (y + 1)² = 5",
                "(x − 2)² + (y + 1)² = 25",
                "(x + 2)² + (y − 1)² = 25",
                "(x − 5)² + (y − 3)² = 25"
              ],
              "correctIndex": 1,
              "explanations": [
                "The center is placed correctly but the right side holds r rather than r². The radius here is 5, so the constant must be 25.",
                "Correct. The radius is the distance from the center to the given point: √((5−2)² + (3−(−1))²) = √(9 + 16) = √25 = 5. With center (2, −1), the equation is (x − 2)² + (y + 1)² = 5² = 25.",
                "This reverses the signs of the center coordinates. Standard form subtracts the center, so a center of (2, −1) produces (x − 2) and (y + 1).",
                "This centers the circle on the point it passes through rather than on the given center. The two roles are not interchangeable."
              ],
              "hint": "Find the radius with the distance formula first, then place the center into standard form — watching both the signs and the squaring."
            },
            {
              "id": "ccp-017",
              "topic": "Perpendicular Bisector Construction",
              "difficulty": "hard",
              "stem": "In the classical compass-and-straightedge construction of a perpendicular bisector, arcs of equal radius are drawn from both endpoints of a segment. Why does the line through the two arc intersections bisect the segment perpendicularly?",
              "choices": [
                "Because the arcs are drawn with a radius exactly half the segment.",
                "Because each intersection point is equidistant from both endpoints, and that locus is the perpendicular bisector.",
                "Because the two arcs are congruent circles.",
                "Because the compass width is arbitrary."
              ],
              "correctIndex": 1,
              "explanations": [
                "The radius need not be half the segment — in fact it must be MORE than half or the arcs never meet. Any radius above that threshold works equally well, which is a hint that the specific value is not what makes the construction succeed.",
                "Correct. Each intersection point sits at the same distance from both endpoints, because both arcs were struck with the same compass setting. The set of all points equidistant from two fixed points IS the perpendicular bisector, so two such points determine it.",
                "The arcs being congruent is what makes each intersection equidistant from the endpoints, so this is a necessary ingredient — but it is not the reason on its own. The conclusion follows from the equidistance locus, not from congruence as such.",
                "The compass width is not entirely arbitrary; it must exceed half the segment length for the arcs to intersect at all. And even granting a valid width, arbitrariness explains nothing about why the result is perpendicular."
              ],
              "hint": "Ask what property the two intersection points share, then recall which locus is defined by exactly that property."
            },
            {
              "id": "ccp-018",
              "topic": "Reasoning: Identify the Conic",
              "difficulty": "hard",
              "stem": "An equation has both x² and y² terms with the SAME coefficient and opposite... no, the same sign, and no xy term. What conic is it, and what would change that?",
              "choices": [
                "A parabola; adding a y² term would make it a circle.",
                "A circle. If the coefficients were unequal but still the same sign it would be an ellipse, and with opposite signs a hyperbola.",
                "A hyperbola, because two squared terms always separate the branches.",
                "It cannot be determined without the constant term."
              ],
              "correctIndex": 1,
              "explanations": [
                "A parabola has only ONE squared variable. Two squared terms rule it out immediately.",
                "Correct. Equal coefficients with the same sign give a circle; unequal but same sign gives an ellipse; opposite signs give a hyperbola; only one squared term gives a parabola.",
                "Opposite signs produce a hyperbola. The same sign keeps the curve closed.",
                "The constant affects the size, and can make the graph empty or a single point, but the TYPE is decided by the squared terms."
              ],
              "hint": "Compare the two squared coefficients: same or different, and same sign or opposite?",
              "kind": "reasoning"
            },
            {
              "id": "ccp-019",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "A student writes that x² + y² = −16 is a circle of radius −4. What is actually true?",
              "choices": [
                "It is a circle of radius 4, since radius is always taken positive.",
                "No real graph exists — a sum of two squares cannot be negative.",
                "It is a hyperbola.",
                "It is a single point at the origin."
              ],
              "correctIndex": 1,
              "explanations": [
                "Flipping the sign changes the equation into a different one. As written, nothing satisfies it.",
                "Correct. x² and y² are each greater than or equal to zero, so their sum can never be −16. The equation has no real solutions and therefore no graph.",
                "A hyperbola needs opposite signs on the squared terms. Both are positive here.",
                "x² + y² = 0 is the single point at the origin. A negative right-hand side gives nothing at all."
              ],
              "kind": "reasoning"
            }
          ]
        },
        {
          "id": "probability-vectors",
          "title": "Geometric Probability & Vectors",
          "blurb": "Probability as a ratio of lengths, angles and areas, and vectors as magnitude-and-direction — components, sums, scaling, and resultants.",
          "meta": "19 questions · easy → hard",
          "categories": [
            "probability",
            "vectors"
          ],
          "questions": [
            {
              "id": "pvc-001",
              "topic": "Geometric Probability",
              "difficulty": "medium",
              "stem": "A circular target of radius 2 is painted at the centre of a square board of side 8. A dart lands at a uniformly random point on the board. What is the probability it lands inside the circle?",
              "choices": [
                "π/16",
                "π/4",
                "4π/8",
                "π/64"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Geometric probability is a ratio of measures: circle area over square area = π(2²)/8² = 4π/64 = π/16 ≈ 0.196.",
                "π/4 is the ratio for a circle INSCRIBED in the square — radius 4, not 2. This circle is much smaller than the board.",
                "4π/8 compares the circle's area to the square's SIDE rather than its area. An area must be divided by an area.",
                "π/64 uses the radius 2 without squaring it, or divides 4π by 64 incorrectly. 4π/64 reduces to π/16."
              ],
              "hint": "Favourable area over total area. Square both dimensions before dividing.",
              "diagram": "<svg viewBox=\"0 0 240 168\" width=\"100%\" style=\"max-width:240px; height:auto; display:block; margin:0 auto;\" role=\"img\"><polygon points=\"60,20 180,20 180,140 60,140\" fill=\"none\" stroke=\"#7c3aed\" stroke-width=\"2\" stroke-linejoin=\"round\"/><circle cx=\"120\" cy=\"80\" r=\"30\" fill=\"none\" stroke=\"#c0392b\" stroke-width=\"2\"/><line x1=\"120\" y1=\"80\" x2=\"150\" y2=\"80\" stroke=\"#c0392b\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"128\" y=\"76\" fill=\"#c0392b\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">2</text><text x=\"112\" y=\"158\" fill=\"#6b6b76\" font-size=\"12\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">8</text></svg>",
              "diagramCaption": "Uniform random point on the square. Not drawn to scale"
            },
            {
              "id": "pvc-002",
              "topic": "Geometric Probability",
              "difficulty": "easy",
              "stem": "A point is chosen at random on a segment from 0 to 10. What is the probability that it lies between 2 and 5?",
              "choices": [
                "3/10",
                "1/2",
                "7/10",
                "3/5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. On a one-dimensional region the measure is length: the favourable interval is 5 − 2 = 3 long, out of a total of 10, so the probability is 3/10.",
                "1/2 would be the answer for an interval of length 5, such as 0 to 5.",
                "7/10 is the probability of landing OUTSIDE the interval, 1 − 3/10.",
                "3/5 divides 3 by 5 rather than by the whole length 10."
              ],
              "hint": "Favourable length over total length. Watch that the interval's length is the difference, not its upper endpoint."
            },
            {
              "id": "pvc-003",
              "topic": "Geometric Probability",
              "difficulty": "medium",
              "stem": "A spinner is divided into sectors. What is the probability of landing in a sector with a central angle of 120°?",
              "choices": [
                "1/3",
                "1/2",
                "120/100",
                "2/3"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. For a spinner the measure is angle: 120°/360° = 1/3.",
                "1/2 would need a 180° sector.",
                "120/100 is greater than 1, which no probability can be. The denominator is 360, not 100.",
                "2/3 is the probability of landing OUTSIDE that sector, 240°/360°."
              ],
              "hint": "The whole spinner is 360°. What fraction of it is this sector?"
            },
            {
              "id": "pvc-004",
              "topic": "Geometric Probability",
              "difficulty": "hard",
              "stem": "Two concentric circles have radii 3 and 5. A point is chosen at random inside the larger circle. What is the probability it falls in the ring between them?",
              "choices": [
                "16/25",
                "2/5",
                "9/25",
                "4/5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The ring's area is π(5²) − π(3²) = 25π − 9π = 16π, and the whole disc is 25π, so the probability is 16π/25π = 16/25 = 0.64.",
                "2/5 is the ratio of the RADII difference to the outer radius, (5−3)/5. Probability compares areas, not radii.",
                "9/25 is the probability of landing INSIDE the smaller circle, which is the complement of the answer.",
                "4/5 does not correspond to either region. Note that squaring the radii is what makes this ring bigger than a radius comparison suggests."
              ],
              "hint": "Subtract the inner area from the outer, then divide by the outer. The π cancels."
            },
            {
              "id": "pvc-005",
              "topic": "Vectors",
              "difficulty": "easy",
              "stem": "What is the magnitude of the vector ⟨3, 4⟩?",
              "choices": [
                "5",
                "7",
                "12",
                "25"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Magnitude is the Pythagorean length of the arrow: |⟨3, 4⟩| = √(3² + 4²) = √25 = 5.",
                "7 adds the components. They are perpendicular, so they combine by Pythagoras.",
                "12 multiplies them, which gives twice the area of the triangle they span — not a length.",
                "25 is the magnitude squared. Take the root."
              ],
              "hint": "The components are the legs of a right triangle; the magnitude is its hypotenuse.",
              "diagram": "<svg viewBox=\"0 0 170 148\" width=\"100%\" style=\"max-width:170px; height:auto; display:block; margin:0 auto;\" role=\"img\"><line x1=\"40\" y1=\"10\" x2=\"40\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"62\" y1=\"10\" x2=\"62\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"84\" y1=\"10\" x2=\"84\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"106\" y1=\"10\" x2=\"106\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"128\" y1=\"10\" x2=\"128\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"150\" y1=\"10\" x2=\"150\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"120\" x2=\"150\" y2=\"120\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"98\" x2=\"150\" y2=\"98\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"76\" x2=\"150\" y2=\"76\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"54\" x2=\"150\" y2=\"54\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"32\" x2=\"150\" y2=\"32\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"10\" x2=\"150\" y2=\"10\" stroke=\"#eceaf2\" stroke-width=\"1\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"120\" x2=\"150\" y2=\"120\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"10\" x2=\"40\" y2=\"120\" stroke=\"#6b6b76\" stroke-width=\"1.4\" stroke-linecap=\"round\"/><line x1=\"40\" y1=\"120\" x2=\"106\" y2=\"32\" stroke=\"#c0392b\" stroke-width=\"2.4\" stroke-linecap=\"round\"/><polygon points=\"106,32 96,42 112,44\" fill=\"#c0392b\"/><line x1=\"40\" y1=\"120\" x2=\"106\" y2=\"120\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><line x1=\"106\" y1=\"120\" x2=\"106\" y2=\"32\" stroke=\"#6b6b76\" stroke-width=\"1.5\" stroke-dasharray=\"5 4\" stroke-linecap=\"round\"/><text x=\"66\" y=\"138\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">3</text><text x=\"114\" y=\"82\" fill=\"#6b6b76\" font-size=\"11\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">4</text><text x=\"56\" y=\"76\" fill=\"#c0392b\" font-size=\"13\" font-family=\"system-ui, sans-serif\" font-weight=\"600\" paint-order=\"stroke\" stroke=\"#ffffff\" stroke-width=\"3.5\" stroke-linejoin=\"round\">?</text></svg>",
              "diagramCaption": "The vector as an arrow from the origin. Not drawn to scale"
            },
            {
              "id": "pvc-006",
              "topic": "Vectors",
              "difficulty": "easy",
              "stem": "Add the vectors ⟨2, 3⟩ and ⟨5, −1⟩.",
              "choices": [
                "⟨7, 2⟩",
                "⟨7, 4⟩",
                "⟨10, −3⟩",
                "⟨−3, 4⟩"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Vectors add component by component: ⟨2 + 5, 3 + (−1)⟩ = ⟨7, 2⟩.",
                "⟨7, 4⟩ adds the y-components as 3 + 1, missing the negative sign on the second one.",
                "⟨10, −3⟩ multiplies the components instead of adding them.",
                "⟨−3, 4⟩ subtracts the first from the second rather than adding."
              ],
              "hint": "Add x to x and y to y, minding the sign on the −1."
            },
            {
              "id": "pvc-007",
              "topic": "Vectors",
              "difficulty": "medium",
              "stem": "A vector runs from the point (2, 3) to the point (7, 1). What is its component form?",
              "choices": [
                "⟨5, −2⟩",
                "⟨−5, 2⟩",
                "⟨9, 4⟩",
                "⟨5, 2⟩"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Subtract the tail from the head: ⟨7 − 2, 1 − 3⟩ = ⟨5, −2⟩. The negative y-component says the vector points downward.",
                "⟨−5, 2⟩ subtracts in the opposite order, giving the vector from (7, 1) back to (2, 3) — the same arrow reversed.",
                "⟨9, 4⟩ adds the coordinates instead of subtracting.",
                "⟨5, 2⟩ has the right x but drops the sign on y. Going from y = 3 to y = 1 is a decrease."
              ],
              "hint": "Head minus tail. The sign of each component tells you which way it points."
            },
            {
              "id": "pvc-008",
              "topic": "Vectors",
              "difficulty": "medium",
              "stem": "What is 3⟨2, −4⟩?",
              "choices": [
                "⟨6, −12⟩",
                "⟨5, −1⟩",
                "⟨6, −4⟩",
                "⟨2, −12⟩"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Scalar multiplication scales every component: ⟨3(2), 3(−4)⟩ = ⟨6, −12⟩. The direction is unchanged and the magnitude is tripled.",
                "⟨5, −1⟩ adds 3 to each component. Scalar multiplication multiplies; adding a number to a vector is not defined.",
                "⟨6, −4⟩ scales only the x-component, which would change the vector's direction as well as its length.",
                "⟨2, −12⟩ scales only the y-component, with the same problem."
              ],
              "hint": "Multiply both components. If only one changes, the arrow points somewhere new — which scaling must not do."
            },
            {
              "id": "pvc-009",
              "topic": "Vectors",
              "difficulty": "hard",
              "stem": "What is the direction angle of the vector ⟨1, √3⟩, measured counterclockwise from the positive x-axis?",
              "choices": [
                "60°",
                "30°",
                "45°",
                "120°"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. tan θ = y/x = √3/1 = √3, so θ = 60°. The vector lies in the first quadrant, where the inverse tangent needs no adjustment. Its components 1 and √3 are the short and long legs of a 30-60-90 triangle.",
                "30° is the angle whose tangent is 1/√3 — the vector ⟨√3, 1⟩, with the components swapped.",
                "45° would need equal components, tan θ = 1.",
                "120° would put the vector in the second quadrant, which needs a negative x-component. Here x = 1 is positive."
              ],
              "hint": "Take the inverse tangent of y over x, then check which quadrant the signs put you in."
            },
            {
              "id": "pvc-010",
              "topic": "Vectors",
              "difficulty": "medium",
              "stem": "A boat heads due east at 12 km/h while a current pushes it due north at 5 km/h. What is its resultant speed?",
              "choices": [
                "13 km/h",
                "17 km/h",
                "7 km/h",
                "√17 km/h"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The two velocities are perpendicular, so they add as a right triangle: √(12² + 5²) = √169 = 13 km/h. The 5-12-13 triple.",
                "17 adds the speeds. That would only be right if both pushed in the SAME direction.",
                "7 subtracts them, which would apply if they were directly opposed.",
                "√17 adds the components before squaring. Each has to be squared first."
              ],
              "hint": "East and north are perpendicular. Draw the two arrows head to tail and measure the closing side."
            },
            {
              "id": "pvc-011",
              "topic": "Vectors",
              "difficulty": "medium",
              "stem": "Two vectors are described as equal. What must be true?",
              "choices": [
                "They have the same magnitude and the same direction",
                "They start at the same point",
                "They have the same magnitude only",
                "They lie on the same line"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. A vector is defined by magnitude and direction alone — its position is not part of it. Two arrows anywhere on the plane are the same vector if they are the same length and point the same way.",
                "Starting point is exactly what a vector does NOT record. Translating an arrow leaves the vector unchanged, which is what makes vector addition position-independent.",
                "Same magnitude alone is not enough: ⟨3, 0⟩ and ⟨0, 3⟩ both have magnitude 3 and are not equal.",
                "Lying on the same line allows opposite directions — ⟨2, 0⟩ and ⟨−2, 0⟩ are collinear but not equal."
              ],
              "hint": "List everything a vector actually encodes. Is location one of them?"
            },
            {
              "id": "pvc-012",
              "topic": "Geometric Probability",
              "difficulty": "medium",
              "stem": "A 10 cm segment is cut at a random point. What is the probability that the shorter piece is less than 2 cm?",
              "choices": [
                "2/5",
                "1/5",
                "1/2",
                "4/5"
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. The shorter piece is under 2 cm when the cut falls within 2 cm of EITHER end — that is the interval from 0 to 2 or from 8 to 10, a favourable length of 2 + 2 = 4 out of 10, giving 4/10 = 2/5.",
                "1/5 counts only one end. The condition is symmetric, so both ends qualify.",
                "1/2 would need a favourable length of 5, which corresponds to a different condition.",
                "4/5 is the complement region measured wrongly; the probability that the shorter piece is at least 2 cm is 6/10 = 3/5, not 4/5."
              ],
              "hint": "Two separate stretches of the segment satisfy the condition, one at each end. Add both before dividing."
            },
            {
              "id": "pvc-013",
              "topic": "Geometric Probability (1D)",
              "difficulty": "medium",
              "stem": "A point is chosen uniformly at random on a segment of length 12. What is the probability it lands within a sub-segment of length 3?",
              "choices": [
                "1/3",
                "1/4",
                "3",
                "1/12"
              ],
              "correctIndex": 1,
              "explanations": [
                "1/3 comes from dividing 12 by 3 and inverting incorrectly, or from using a sub-segment of length 4. Divide the favorable length by the total length as written.",
                "Correct. In continuous sample spaces the counting formula fails, so probability is measured rather than counted: P = length of favorable region / length of whole region = 3/12 = 1/4.",
                "3 is the favorable length itself. A probability must fall between 0 and 1, so any answer above 1 signals that the division was skipped.",
                "1/12 would be the probability for a favorable region of length 1. The favorable region here is 3 units long."
              ],
              "hint": "When outcomes are uncountable, swap counting for measuring. The formula keeps its shape but changes what it counts."
            },
            {
              "id": "pvc-014",
              "topic": "Probability Zero",
              "difficulty": "hard",
              "stem": "A point is chosen uniformly at random on a segment. What is the probability it lands on one specific pre-chosen point, and what does that value mean?",
              "choices": [
                "0, and that outcome is impossible",
                "0, but the outcome is still possible",
                "Undefined, since a point has no length",
                "1 divided by the number of points on the segment"
              ],
              "correctIndex": 1,
              "explanations": [
                "The probability is indeed 0, but 'impossible' is the wrong reading. Some point certainly gets chosen, so outcomes of probability zero do occur — this is the key conceptual break between finite and continuous sample spaces.",
                "Correct. A single point has length 0, so P = 0/ℓ(S) = 0. Yet the process always selects some point, so probability-zero events are not impossible ones. In continuous spaces, 'probability 0' means 'negligible in measure', not 'cannot happen'.",
                "It is well defined: the measure of a single point is 0, and dividing by the segment's positive length gives exactly 0. Nothing here is ill-posed.",
                "The number of points on a segment is uncountably infinite, so this expression is not a real number. It is precisely the breakdown of the counting formula that forces the shift to measuring lengths."
              ],
              "hint": "Compute the value first, then think carefully about whether 'probability zero' and 'impossible' really mean the same thing."
            },
            {
              "id": "pvc-015",
              "topic": "Complement Rule",
              "difficulty": "medium",
              "stem": "A fair coin is flipped 3 times. What is the probability of getting at least one head?",
              "choices": [
                "3/8",
                "1/2",
                "7/8",
                "1/8"
              ],
              "correctIndex": 2,
              "explanations": [
                "3/8 is the probability of exactly one head. 'At least one' covers exactly one, exactly two, and exactly three.",
                "1/2 is the probability of a head on a single flip. Across three flips the chance of seeing at least one is considerably higher.",
                "Correct. Use the complement: the only way to fail is all tails, with probability (1/2)³ = 1/8. So P(at least one head) = 1 − 1/8 = 7/8. The complement rule is the standard tool for 'at least one' problems precisely because it replaces three cases with one.",
                "1/8 is the probability of NO heads — the complement itself, not the event asked about."
              ],
              "hint": "'At least one' has many favorable cases but only one unfavorable case. Count the easy side and subtract."
            },
            {
              "id": "pvc-016",
              "topic": "Permutations vs Combinations",
              "difficulty": "medium",
              "stem": "From a class of 10 students, how many different 3-person committees can be formed?",
              "choices": [
                "720",
                "120",
                "30",
                "1000"
              ],
              "correctIndex": 1,
              "explanations": [
                "720 is ₁₀P₃ = 10!/7!, the number of ordered ARRANGEMENTS. A committee has no internal ranking, so the three orderings of the same trio should not be counted separately.",
                "Correct. A committee is unordered, so this is a combination: ₁₀C₃ = 10!/(3!·7!) = 120. Note that 720/3! = 120 — the permutation count divided by the 6 ways to order each committee.",
                "30 is 10 × 3, which multiplies the two given numbers directly. Counting problems of this kind need factorials, not a single product.",
                "1000 is 10³, which would count selections made WITH replacement and WITH order — allowing the same student to be picked three times."
              ],
              "hint": "Decide whether swapping two chosen people creates a different outcome. That single question separates the two formulas."
            },
            {
              "id": "pvc-017",
              "topic": "Vector Magnitude",
              "difficulty": "easy",
              "stem": "Find the magnitude of the vector ⟨5, −12⟩.",
              "choices": [
                "7",
                "13",
                "17",
                "√17"
              ],
              "correctIndex": 1,
              "explanations": [
                "7 subtracts the components, 12 − 5. Magnitude combines them through the Pythagorean Theorem, not by subtraction.",
                "Correct. The magnitude is √(5² + (−12)²) = √(25 + 144) = √169 = 13 — the 5-12-13 triple. Squaring removes the sign, so the negative component contributes exactly as a positive one would.",
                "17 adds the absolute values of the components, 5 + 12. That is the distance travelled along the axes, not the straight-line length of the vector.",
                "√17 adds the components before squaring rather than squaring each first. Each component is squared individually, then summed."
              ],
              "hint": "A vector's magnitude is the hypotenuse of the right triangle its components form. The sign of a component does not survive squaring."
            },
            {
              "id": "pvc-018",
              "topic": "Reasoning: Error Analysis",
              "difficulty": "medium",
              "stem": "A student adds vectors (3, 4) and (1, 2) by adding their magnitudes: 5 + √5. Why is that wrong?",
              "choices": [
                "Vectors add COMPONENTWISE, giving (4, 6), whose magnitude is √52 — not the sum of the magnitudes.",
                "Magnitudes can never be added.",
                "The student should have multiplied the magnitudes.",
                "Nothing is wrong."
              ],
              "correctIndex": 0,
              "explanations": [
                "Correct. Vector addition is componentwise: (3+1, 4+2) = (4, 6), with magnitude √(16+36) = √52. Magnitudes only add when the vectors point in exactly the same direction.",
                "Magnitudes CAN be added — but only when the vectors are parallel and in the same direction, which these are not.",
                "Multiplication of magnitudes is part of the dot product, which produces a scalar rather than a resultant vector.",
                "Adding magnitudes overstates the resultant whenever the vectors differ in direction."
              ],
              "hint": "Direction matters. Two vectors pointing different ways partly cancel.",
              "kind": "reasoning"
            },
            {
              "id": "pvc-019",
              "topic": "Reasoning: Probability",
              "difficulty": "hard",
              "stem": "A dart lands at random on a square target with a circle inscribed in it. A student says the chance of hitting the circle is 1/2 \"because the circle covers about half\". What is the actual reasoning?",
              "choices": [
                "It is 1/2; the circle covers half the square.",
                "Geometric probability is a ratio of AREAS: πr² over (2r)², which is π/4 ≈ 0.785.",
                "It is 1/4, since the circle touches four sides.",
                "It cannot be found without the radius."
              ],
              "correctIndex": 1,
              "explanations": [
                "The estimate is well short. An inscribed circle covers considerably more than half its square.",
                "Correct. Geometric probability compares areas. With side 2r, the square has area 4r² and the circle πr², so the probability is π/4 ≈ 78.5%.",
                "The number of tangent points has nothing to do with the area covered.",
                "The radius cancels out, which is why the answer is the same for every square."
              ],
              "kind": "reasoning"
            }
          ]
        }
      ]
    }
  }
}