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Team Selection Test for EGMO 2024

Turkey 2024 geometry

Problem

Let be a triangle, be its circumcircle and be its incentre. Let the line meet at and at for the second time. The line meet at and at for the second time. Let the circumcircles of and meet at for the second time. Prove that the lines , , are concurrent.

problem
Solution
Let . Since is the Miquel point of the quadrilateral we get that and are conicyclic. Therefore, we have and since we get that quadrilateral is cyclic. Similarly the quadrilateral is also cyclic. By using of the radical axis theorem on the circles , and we get the desired concurrency.

Techniques

Miquel pointRadical axis theoremAngle chasingTriangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circle