Find the number of triangles in the given figure.

20232024

Find the number of triangles in the given figure.

  1. A.

    18

  2. B.

    20

  3. C.

    28

  4. D.

    34

Attempted by 2 students.

Show answer & explanation

Correct answer: C

Concept: To count triangles in a figure built from several crossing lines, work level by level: first count the simplest triangles made of a single smallest region, then the triangles formed by combining exactly two adjacent regions, then exactly three, and so on -- then add every level together, being careful not to count any triangle twice or to miss a composite shape.

Application: Label the figure as shown above, with O at the centre and E, F, G, H, I, J, K, L marking the points where the lines cross to form the smaller regions inside the outer points A, B, C, D.

  1. Simplest (single-region) triangles: AEI, AIH, BEJ, BJF, CFK, CKG, DGL, DLH, EOJ, FOJ, FOG, LOG, HOL and HOE -- 14 triangles.

  2. Triangles formed by combining exactly two adjacent regions: EAH, FBE, BEO, EOF, BFO, FCG, GDH, HOD, HOG and GOD -- 10 triangles.

  3. Triangles formed by combining exactly three adjacent regions: EFH, EHG, FGH and EFG -- 4 triangles.

  4. Total = 14 + 10 + 4 = 28 triangles.

Cross-check: As an independent consistency check, recount the triangle names actually listed at each level: level 1 lists exactly 14 distinct names, level 2 lists exactly 10 distinct names, and level 3 lists exactly 4 distinct names, with no name repeated at any level or across levels. This rules out a double-count or a dropped entry. The level-by-level method (single-region, then two-region, then three-region combinations) is itself the standard, exhaustive way to count triangles in a figure built from overlapping regions, so the confirmed recount 14 + 10 + 4 = 28 is complete and accurate.

Hence, the total number of triangles in the given figure is 28.

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