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Junior Macedonian Mathematical Olympiad

North Macedonia geometry

Problem

A regular hexagon with side-length is given. Inside the hexagon points are given so that no three of them are collinear. The hexagon is split into triangles, so that each of the given points and each vertex of the hexagon is a vertex of one such triangle. The triangles into which the hexagon is split have no common interior point. Prove that, among them, there exists one triangle whose area is not greater than .
Solution
First we determine the total number of splitting triangles into which the hexagon is split. Let be one arbitrary point of the interior points. The sum of all angles at the point is (the sum of all angles at of all triangles having that point as a vertex). On the other hand, the sum of all angles at a vertex of the hexagon is . Since the sum of angles in every triangle is , the number of splitting triangles is: .

Let us assume the contrary to the statement, i.e. that the area of each of the splitting triangles is greater than . Then the sum of the areas of all the splitting triangles is greater than , which is impossible since the area of the given hexagon is .

Techniques

Angle chasingPigeonhole principle