IMO 1966 LL CZS41

If A1A2 . . . An is a regular n-gon (n \geq3), how many different

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IMO 1966 LL CZS41

Origin: CZS

Problem

If A1A2 . . . An is a regular n-gon (n \geq3), how many different obtuse triangles AiAjAk exist?