Browse · MathNet
PrintTeam 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 .

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