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Team Selection Test for JBMO

Turkey geometry

Problem

In a convex quadrilateral , the diagonals intersect at , and . Let be the foot of the perpendicular from to and be the second intersection point of the circumcircle of triangle and the line segment . Find .

problem
Solution
Let be the foot of the perpendicular from to the line . Then we obtain which means that . Since the points are concyclic, we have . Since the points are also concyclic we have and hence and therefore the points are concyclic. So, .

Final answer
22.5°

Techniques

Cyclic quadrilateralsAngle chasingDistance chasing