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Geometry Difficulty 5.6 AIME, harder Prove it India

Suppose 20162016 points of the circumference of a circle are coloured red and the remaining points are coloured blue. Given any natural number n3n \ge 3, prove that there is a regular nn-sided polygon all of whose vertices are blue.

Solution

Let A1,A2,,A2016A_1, A_2, \dots, A_{2016} be 20162016 points on the circle which are coloured red and the remaining blue. Let n3n \ge 3 and let B1,B2,,BnB_1, B_2, \dots, B_n be a regular nn-sided polygon inscribed in this circle with the vertices chosen in anti-clock-wise direction. We place B1B_1 at A1A_1. (It is possible, in this position, some other BB's also coincide with some other AA's.) Rotate the polygon in anti-clock-wise direction gradually till some BB's coincide with (an equal number of) AA's second time. We again rotate the polygon in the same direction till some BB's coincide with an equal number of AA's third time, and so on until we return to the original position, i.e., B1B_1 at A1A_1. We see that the number of rotations will not be more than 2016×n2016 \times n, that is, at most these many times some BB's would have coincided with an equal number of AA's. Since the interval (0,360)(0, 360^\circ) has infinitely many points, we can find a value α(0,360)\alpha^\circ \in (0, 360^\circ) through which the polygon can be rotated from its initial position such that no BB coincides with any AA. This gives a nn-sided regular polygon having only blue vertices.

Alternate Solution:

Consider a regular 2017×n2017 \times n-gon on the circle; say, A1A2A3A2017nA_1A_2A_3\cdots A_{2017n}. For each jj, 1j20171 \le j \le 2017, consider the points {Ak:kj(mod2017)}\{A_k : k \equiv j \pmod{2017}\}. These are the vertices of a regular nn-gon, say SjS_j. We get 20172017 regular nn-gons; S1,S2,,S2017S_1, S_2, \dots, S_{2017}. Since there are only 20162016 red points, by pigeon-hole principle there must be some nn-gon among these 20172017 which does not contain any red point. But then it is a blue nn-gon.

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