Middle European Mathematical Olympiad 2023 Problem T-6
Let be an acute triangle with . Let be the center of the -excircle of . Let be the projection of on line . The internal bisectors of angles and intersect lines and at and , respectively. Segments and intersect at . Let be the projection of on line . Prove that the internal angle bisector of is perpendicular to line .
Remark. The -excircle of the triangle is the circle outside the triangle which is tangent to the lines , , and the line segment .