IMO 1966 LL BUL32
The sides a, b, c of a triangle ABC form an arithmetic progression;
IMO 1966 LL BUL32
Origin: BUL
Problem
The sides a, b, c of a triangle ABC form an arithmetic progression; the sides of another triangle A1B1C1 also form an arithmetic progression. Suppose that \angleA = \angleA1. Prove that the triangles ABC and A1B1C1 are similar.