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Combinatorics Difficulty 4.4 AIME Prove it Taiwan

Given a parallelogram, all four of its angles are rational multiples of 360°360°. Question: is it possible to color every point in the coordinate plane except the origin in black or white, such that whenever some three points together with the origin form a shape similar to this quadrilateral, then these three points must not all be the same color?

Solution

Yes, it is possible. First, if the parallelogram is not a rhombus, let t>1t > 1 be the ratio of the long side to the short side. For any point in the plane, if the distance from that point to the origin lies within [t2k,t2k+1)[t^{2k}, t^{2k+1}) (for any integer kk), color it black; otherwise color it white. In this way, the two points of the parallelogram closest to the origin must be of different colors.

Next, consider the case where the parallelogram is a rhombus. Let its interior angle that is less than or equal to 90°90° have measure 2a2a, i.e., a45°a \le 45°. Then the angle between a side and a diagonal is either aa or 90°a90° - a. Now color as follows: measuring the elevation angle from the center with respect to the xx-axis direction, divide the interval [0°,90°)[0°, 90°) into [0,a)[0, a), [a,2a)[a, 2a), ..., [ka,90°)[ka, 90°), and color the points whose argument falls in the first interval black, the second white, alternating black and white in this manner. For the colors of angles beyond 90°90°, follow this rule: if the arguments of two points differ by 90°90°, then these two points are the same color. In this way, any three consecutive points (in counterclockwise order, labeled A,B,CA, B, C) whose arguments differ successively by aa cannot all be the same color; in particular: when the elevation angles of A,BA, B lie in [0,90°)[0, 90°), A,BA, B are of different colors; if the elevation angle of AA lies in [0,90°)[0, 90°) but the elevation angles of B,CB, C lie in [90°,180°)[90°, 180°), then B,CB, C are of different colors. (There cannot be a situation where A,B,CA, B, C lie in three different quadrants, since a45°a \le 45°.) All other situations are symmetric to these. Moreover, differing by 90°a90° - a is equivalent to differing by aa with respect to the coloring, and thus the result is proven.

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Source: MathNet, licensed CC-BY-4.0. Statement translated into English from zh; metadata (topic, difficulty) added by this project.