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Autumn tournament

Bulgaria geometry

Problem

Given an acute triangle with circumcenter . The point on such that and the point is on , such that . The line meets at and is the midpoint of . The circumcircle of meets at . The lines , meet at , . Show that is cyclic.

(Alexander Ivanov)
Solution
Let be the antipode of and let . Since , by Reim's theorem we need being cyclic, or , so we need being cyclic, or that . By shooting lemma, is cyclic, so , so we need being the angle bisector of . Since bisects , it is sufficient to show that .

To use that is cyclic, let ; Reim's implies , which together with the length condition gives that is an isosceles trapezoid. Hence, , which finishes the problem.

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleCyclic quadrilateralsRadical axis theoremAngle chasing