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max numbers of isosceles triangles by 100 collinear point and 1 outside

Source: December 2020 Ukraine Geometry Olympiad X p2 , XI p2

December 20, 2020
geometrycombinatoricscombinatorial geometryisosceles

Problem Statement

On a straight line lie 100100 points and another point outside the line. Which is the biggest the number of isosceles triangles can be formed from the vertices of these 101101 points?