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Saudi Arabia geometry
Problem
The points of the plane have been colored by different colors. We say that a triangle has the color if its three vertices , , have the color . Prove that there are infinitely many triangles with the same color and the same area.

Solution
Consider parallel lines. Each line contains infinitely many points. Since the number of the colors is finite, by the pigeonhole principle, there exist on each line infinitely many points of the same color. Choose for each line one color for which there exist infinitely many points. Since there are colors and lines, by the pigeonhole principle there exist at least two lines , for which the same color has been chosen. Choose two points , from the first line of this color . Choose infinitely many points , , from the second line of this color . Triangles , , are all of the same color and have the same area since , are parallel.
Techniques
TrianglesCombinatorial GeometryPigeonhole principle