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Geometry Difficulty 4.9 AIME Prove it Brazil

Does there exist a set of n>2n > 2 points in the plane such that no three are collinear and the circumcenter of any three points of the set is also in the set?

Solution

No, it's not possible. Let SS be the set of nn points. Consider the smallest circumcircle ω\omega of all triangles determined by the nn points. Let A,B,CSA, B, C \in S be three points on ω\omega and let OO be its center. If one of the central angles AOB\angle AOB, BOC\angle BOC, COA\angle COA is less than 120120^\circ then the circumcircle of the correspondent triangle is smaller than ω\omega. Since the sum of the three central angles is 360360^\circ, AOB=BOC=COA=120\angle AOB = \angle BOC = \angle COA = 120^\circ. The circumcenters of AOBAOB, BOCBOC and COACOA are the midpoints of the arcs ABAB, BCBC, CACA, and then there are six points from SS on ω\omega forming a regular hexagon. Take OO and two of these points and obtain a circumcircle smaller than ω\omega, which is a contradiction. So there is no such set.

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