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Geometry Difficulty 5.3 AIME, harder Prove it Romania

Given a positive integer nn, does there exist a planar polygon and a point in its plane such that every line through that point meets the boundary of the polygon at exactly 2n2n points?

Solution

The answer is in the affirmative. To describe the configuration, fix a coordinate frame and let a0,a1,,a4n1a_0, a_1, \dots, a_{4n-1} be real numbers such that a0>0>a2>a4n2>a4>a4n4>>a2n2>a2n+2>a2na_0 > 0 > a_2 > a_{4n-2} > a_4 > a_{4n-4} > \dots > a_{2n-2} > a_{2n+2} > a_{2n}, and a2n+1<a2n+3<<a4n1<0<a1<a3<<a2n1a_{2n+1} < a_{2n+3} < \dots < a_{4n-1} < 0 < a_1 < a_3 < \dots < a_{2n-1}. Setting A2k=0×a2kA_{2k} = 0 \times a_{2k} and A2k+1=a2k+1×0A_{2k+1} = a_{2k+1} \times 0, k=0,,2n1k = 0, \dots, 2n-1, the polygon A0A1A4n1A_0A_1 \dots A_{4n-1} and the origin satisfy the condition in the statement: the x-axis (respectively, y-axis) contains all vertices of odd (respectively, even) rank and no other points on the boundary; and every line through the first and third (respectively, second and fourth) quadrants crosses the sides A0A1A_0A_1 and A2n+k1A2n+kA_{2n+k-1}A_{2n+k} (respectively, A4n1A0A_{4n-1}A_0 and AkAk+1A_kA_{k+1}), k=1,,2n1k = 1, \dots, 2n-1, and no other side, since the remaining sides all lie in the other two quadrants.

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