What is the number of straight lines and the number of triangles in the given…
2025
What is the number of straight lines and the number of triangles in the given figure.

- A.
10 straight lines and 34 triangles
- B.
9 straight lines and 34 triangles
- C.
9 straight lines and 36 triangles
- D.
10 straight lines and 36 triangles
Show answer & explanation
Correct answer: C
Concept: In "counting figures" reasoning problems, straight lines are counted as maximal continuous segments grouped by orientation (horizontal, vertical, slanting), and triangles are counted systematically by order — the number of elementary regions each triangle is built from — starting at the smallest single-region triangles and going up to the one largest triangle spanning the whole figure, then summing across every order.
Application: Label the figure as shown, with apex A; the upper line through D, E, F; the base line through B, G, H, I, C; and the points J, K, L formed further down where the diagonals and the vertical line cross.

Counting the straight lines by orientation:
Orientation | Lines | Count |
|---|---|---|
Horizontal | DF, BC | 2 |
Vertical | DG, AH, FI | 3 |
Slanting | AB, AC, BF, DC | 4 |
Total straight lines = 2 + 3 + 4 = 9.
Counting the triangles by order (the number of elementary regions each triangle is built from):
Order 1 (simplest, single-region) triangles: ADE, AEF, DEK, EFK, DJK, FLK, DJB, FLC, BJG, LIC — 10 triangles.
Order 2 (two-region) triangles: ADF, AFK, DFK, ADK, DKB, FCK, BKH, KHC, DGB, FIC — 10 triangles.
Order 3 (three-region) triangles: DFJ, DFL — 2 triangles.
Order 4 (four-region) triangles: ABK, ACK, BFI, CDG, DFB, DFC, BKC — 7 triangles.
Order 6 (six-region) triangles: ABH, ACH, ABF, ACD, BFC, CDB — 6 triangles.
Order 12 (the entire figure): ABC — 1 triangle.
Total triangles = 10 + 10 + 2 + 7 + 6 + 1 = 36.
Cross-check: Both tallies check out independently — the line count by orientation (2 + 3 + 4) and the triangle count by order (10 + 10 + 2 + 7 + 6 + 1) each sum correctly on their own, and together they match only the option that states nine straight lines together with thirty-six triangles.