3

Part of 1993 India Regional Mathematical Olympiad

Problems(1)

Source: Indian RMO 1993 Problem 3

A_1, A_2, A_3, \ldots, A_{20}

is a 20 sides regular polygon. How many non-isosceles (scalene) triangles can be formed whose vertices are among the vertices of the polygon but the sides are not the sides of the polygon?

geometrycongruent triangles