Find the number of triangles in the given figure.

2025

Find the number of triangles in the given figure.

  1. A.

    11

  2. B.

    10

  3. C.

    20

  4. D.

    18

Attempted by 6 students.

Show answer & explanation

Correct answer: B

Concept: When counting triangles in a figure built from intersecting chords, first identify every triangle bounded directly by the drawn line segments — these are the elementary (simple) triangles. Then identify every larger triangle formed by joining two or more elementary triangles along a shared segment, provided its own three sides are actually drawn lines. The total number of triangles is the sum of both groups.

Application: Label the figure as shown: A, B, C, D, E lie on the circle with F below chord AE, and G is the point where the two diagonals AC and BE cross inside quadrilateral ABCE.

Simple (elementary) triangles — six in total:

  • ABG, BCG, CGE and AGE — the four triangles formed inside quadrilateral ABCE where its two diagonals AC and BE cross at G.

  • CDE — bounded by sides CD, DE and diagonal CE.

  • AEF — bounded by sides AF, FE and side EA.

Composite triangles — four in total, each formed by joining exactly two of the four elementary triangles inside ABCE across point G (G lies on only one side of each, so a single line splits it into two parts, not three):

Composite triangle

Formed from

ABC

ABG + BCG

ABE

ABG + AGE

BCE

BCG + CGE

ACE

AGE + CGE

Total = 6 simple + 4 composite = 10 triangles.

Cross-check: A quadrilateral with both diagonals drawn always contains exactly 8 triangles (the 4 elementary ones plus the 4 formed by one full diagonal with two sides), so ABCE alone accounts for 8. CDE and AEF sit outside ABCE and share no interior lines with it, adding 2 more independent triangles. 8 + 2 = 10, confirming the count.

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