The diagonal of a convex quadrilateral is the bisector of the angle . Let be the intersection of the side and the circumcircle of the triangle . Let be the intersection of the side and the circumcircle of the triangle . Prove that the segments and intersect in a point.
Solution
Let be the intersection of the segments and . The points and lie on the same circle, so . The diagonal bisects the angle , so . This implies and . We conclude that the points and lie on the same circle.

The points and are concyclic, so and . Since is a cyclic quadrilateral, we have and . So, . We have shown that and , so . This implies that is a cyclic quadrilateral. So, , or . At the same time we have because is the bisector of the angle . Since is a cyclic quadrilateral, we have . We conclude that and the points and are collinear. Hence, and meet at a point.
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