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31st Junior Turkish Mathematical Olympiad

Turkey geometry

Problem

Let be a cyclic quadrilateral and let the incenters of the triangles and be and , respectively. Let the intersection point of the line that passes through and perpendicular to and the line that passes through and perpendicular to be . Prove that .
Solution
Let be the midpoint of arc not containing points . Then, it is well known that hence are concyclic. Therefore we get Similarly is equal to the same value and hence .

Techniques

Cyclic quadrilateralsTriangle centers: centroid, incenter, circumcenter, orthocenter, Euler line, nine-point circleAngle chasing