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Combinatorics Difficulty 6.3 National olympiad Prove it Ukraine

On the plane 2022 points A1,A2,,A2022A_1, A_2, \dots, A_{2022} are given, no three of which lie on the same line. Consider all the angles AiAjAkA_i A_j A_k for the triples of distinct points Ai,Aj,AkA_i, A_j, A_k. What largest number of these angles can be right?

Solution

Consider any point AiA_i and count the number of pairs of points (Aj,Ak)(A_j, A_k) such that AiAjAk=90\angle A_i A_j A_k = 90^\circ. For each point AjA_j there exists at most one point AkA_k (because on the line through AjA_j perpendicular to AiAjA_iA_j there can be at most one point other than AjA_j). Also note that if XX is the point at the largest distance from AiA_i then there can be no point AkA_k with AiXAk=90\angle A_iXA_k = 90^\circ, because then we would have AiAk>AiXA_iA_k > A_iX.

Thus, each point can be a vertex of the hypotenuse in at most 2020 right triangles at these points. Since the hypotenuse of each triangle has two vertices, the total number of these triangles does not exceed 202220202=20221010\frac{2022 \cdot 2020}{2} = 2022 \cdot 1010.

This number can be achieved because, for example, we could take 2022 points on a circle so that they are divided into 1011 pairs so that in each pair the points form a circle diameter. Note that for each diameter there will be exactly 2020 points that form a right triangle with it. Then we have at least 101120201011 \cdot 2020 different right angles.

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