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A 20-gon

Source: Indian RMO 1993 Problem 3

October 23, 2005
geometrycongruent triangles

Problem Statement

Suppose A1,A2,A3,…,A20A_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?
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