Maths Olympiad Prep

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Geometry Difficulty 7.6 National olympiad, round 2 Prove it Saudi Arabia

The points of the plane have been colored by 20132013 different colors. We say that a triangle ABC\triangle ABC has the color XX if its three vertices AA, BB, CC have the color XX. Prove that there are infinitely many triangles with the same color and the same area.

Solution

Consider 20142014 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 20132013 colors and 20142014 lines, by the pigeonhole principle there exist at least two lines 1\ell_1, 2\ell_2 for which the same color C\mathcal{C} has been chosen. Choose two points BB, CC from the first line 1\ell_1 of this color C\mathcal{C}. Choose infinitely many points A1A_1, A2A_2, \ldots from the second line 2\ell_2 of this color C\mathcal{C}. Triangles A1BCA_1BC, A2BCA_2BC, \ldots are all of the same color C\mathcal{C} and have the same area since 1\ell_1, 2\ell_2 are parallel.

Figure 1

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