How many triangles are in a polygon whose sum of its interior angles is 1080°.

2 answers

recall that an n-gon's interior angles sum to

(n-2)*180

so, just solve

(n-2)*180 = 1080
(n-2)*180 = 1080
180n - 360 = 1080
n = 8

A triangle can be drawn by joining any 3 of the 8 vertices.
that would be C(8,3) or 56 such triangles.