Question

In a regular polygon with 15 sides, find the number of triangles

Answer 1

We need 3 sides to make a triangle

so
r = 3
n = 15

(formula to find the number of the triangle is nC3)

15C3

= 15!/(15-3)3!

= 15!/12! × 3!

= 15×14×13×12!/12! × 3×2×1

= 5 × 7 × 13

= 455