In quadrilateral , , and is a right angle. The midpoint of side is , the orthogonal projection of on is , and the orthogonal projection of on is . The second intersection point of circle (with center ) and line is , and the second intersection point of circle and line is . The intersection of lines and is . The center of the Feurbach circle of triangle is . Prove that is a right angle.
KöMaL problem A, 2019
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