Let be a quadrilateral inscribed in a circle . Let the tangent to at intersect the rays and at points and , respectively. A point is chosen inside the triangle so that and . Let be a point on the segment such that . Prove that the lines and intersect at one point.
Solution
Let the segments and cross at and , respectively. Since and is tangent to the circumcircle of , we have
and so the points , and lie on some circle . Similarly, the points , and lie on some circle . We now want to prove that the line is tangent to both and at . Indeed, since , the circles and are tangent to each other at . To prove that lies on their common tangent line at (i.e., on their radical axis), it suffices to check that , or that the quadrilateral is cyclic. This fact follows from
Since , we have . Next, as and are tangent to and , respectively, we obtain
which implies . Next we prove that the five points , and lie on some circle . Indeed, since is tangent to the circle we have
which means that the point lies on the circle (TDK). Similarly, we have . Finally, we prove that . Indeed, using the circles and we conclude that
which means that . Triangles and have pairwise parallel sides, which implies the fact that and are concurrent, as desired. !
Comment 1. There are several variations of the above solution. E.g., after finding circles and , one can notice that there exists a homothety mapping the triangle to the triangle ; the centre of that homothety is . Since
the quadrilateral is inscribed in some circle . We have , so the point lies on .
Finally, by
the points and are symmetric with respect to the diameter of passing through . This yields and , so , and passes through .