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Baltic Way 2023 Shortlist

Baltic Way 2023 geometry

Problem

Let be an acute triangle with . The internal angle bisector of intersects at . Let be the circumcenter of . Let intersect the segment at . Let be the incenter of . Prove that if then .
Solution
Let , , . We have and Therefore, , hence quadrilateral is cyclic. Since is the bisector of , the arcs and are equal. Hence .

Techniques

Triangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleCyclic quadrilateralsAngle chasing