Find the number of triangles in the given figure?
2026
Find the number of triangles in the given figure?

- A.
28
- B.
35
- C.
79
- 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.

One-component triangles (7): AGH, GFO, LFO, DJK, EKP, PEL and IMN.
Two-component triangles (8): GFL, KEL, AMO, NDP, BHN, CMJ, NEJ and HFM.
Three-component triangles (4): IOE, IFP, BIF and CEI.
Four-component triangles (2): ANE and DMF.
Five-component triangles (3): FCK, BGE and ADL.
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.