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KöMaL problem A, 2023

Geometry Difficulty 9.0 IMO level Prove it Hungary

Let points AA, BB, CC, A′A', B′B', C′C' be chosen in the plane such that no three of them are collinear, and let lines AA′AA', BB′BB', CC′CC' be tangent to a given equilateral hyperbola at points AA, BB and CC, respectively. Assume that the circumcircle of A′B′C′A'B'C' is the same as the nine-point circle of triangle ABCABC. Let s(A′)s(A') be the Simson line of point A′A' with respect to the pedal triangle of ABCABC. Let A∗A^* be the intersection of line B′C′B'C' and the perpendicular of s(A′)s(A') through point AA. Points B∗B^* and C∗C^* are defined in a similar manner. Prove that points A∗A^*, B∗B^* and C∗C^* are collinear.

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