Point on side of quadrilateral is such that quadrilaterals and are circumscribed around circles centered at and respectively. Line cuts an isosceles triangle with vertex from angle . Prove that is a cyclic quadrilateral.
Solution
If then the incircles of and have equal radii; now the problem conditions imply that the whole picture is symmetric about the perpendicular from to , and hence is an isosceles trapezoid (or a rectangle). The conclusion in this case is true.

Now suppose that the lines and meet at a point ; we may assume that lies between and . The points and lie on the bisector of the angle . By the problem condition, this angle bisector forms equal angles with the lines and ; this yields . Since and are the incenter of and an excenter of , respectively, we have
so the quadrilateral is cyclic. Next, the same points are an excenter of and the incenter of , respectively, so
this implies the desired cyclicity of the quadrilateral .
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