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Junior Macedonian Mathematical Olympiad

Geometry Difficulty 4.3 AIME Prove it North Macedonia

A regular hexagon with side-length 11 is given. Inside the hexagon mm points are given so that no three of them are collinear. The hexagon is split into triangles, so that each of the given mm 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 334(m+2)\frac{3\sqrt{3}}{4(m+2)}.

Solution

First we determine the total number of splitting triangles into which the hexagon is split. Let AA be one arbitrary point of the interior mm points. The sum of all angles at the point AA is 360∘360^\circ (the sum of all angles at AA 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 120∘120^\circ. Since the sum of angles in every triangle is 180∘180^\circ, the number of splitting triangles is: m⋅360∘+6⋅120∘180∘=2m+4\frac{m \cdot 360^\circ + 6 \cdot 120^\circ}{180^\circ} = 2m+4.

Let us assume the contrary to the statement, i.e. that the area of each of the splitting triangles is greater than 334(m+2)\frac{3\sqrt{3}}{4(m+2)}. Then the sum of the areas of all the splitting triangles is greater than (2m+4)334(m+2)=332(2m+4)\frac{3\sqrt{3}}{4(m+2)} = \frac{3\sqrt{3}}{2}, which is impossible since the area of the given hexagon is 332\frac{3\sqrt{3}}{2}.

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