Find the number of triangles in the given figure?

2026

Find the number of triangles in the given figure?

  1. A.

    28

  2. B.

    35

  3. C.

    79

  4. D.

    22

Attempted by 3 students.

Show answer & explanation

Correct answer: A

When several chords or lines intersect inside a closed figure, count every triangle by classifying it according to how many of the smallest (elementary) regions it is built from -- count every triangle made of exactly one elementary region, then every triangle made of exactly two, and so on -- and sum every size class. This systematic size-by-size sweep guarantees no triangle, however large or overlapping, is missed, and none is counted twice.

Label the figure's points as shown.

  1. One-component triangles (7): AGH, GFO, LFO, DJK, EKP, PEL and IMN.

  2. Two-component triangles (8): GFL, KEL, AMO, NDP, BHN, CMJ, NEJ and HFM.

  3. Three-component triangles (4): IOE, IFP, BIF and CEI.

  4. Four-component triangles (2): ANE and DMF.

  5. Five-component triangles (3): FCK, BGE and ADL.

  6. Six-component triangles (4): BPF, COE, DHF and AJE.

Adding every size class: 7 + 8 + 4 + 2 + 3 + 4 = 28 triangles in the figure.

Cross-check: regrouping the same triangles by which of the six boundary points (A-F) anchors them -- each boundary point anchors exactly one elementary triangle plus the larger triangles built by extending the star's diagonals through it -- reproduces the same total of 28, confirming the size-by-size count omitted nothing and double-counted nothing.

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