IMO Shortlist 1966 problem 41

Given a regular $n$ -gon $A_{1}A_{2}...A_{n}$ (with $n\geq 3$ ) in a plane. How many triangles of the kind $A_{i}A_{j}A_{k}$ are obtuse ?

Izvor: Međunarodna matematička olimpijada, shortlist 1966

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