BALKAN-SL 2015 G6
Let be a diameter of a circle with centre . From an arbitrary point on such that we draw the circles and with diameters and respectively. Let be an exterior common tangent of such that belongs to and belongs to . The point is diametrically opposite to with respect to and the tangent to at the point intersects at the points . If the line of the common chord of the circumcircles of the triangles and intersects the circle at the points and the circle at the point (with closer to ), then prove that .
Dataset text AI-MO AoPS. Quoted with source.
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