In triangle , points lie on sides , respectively. Let denote the circumcircles of triangles , respectively. Given the fact that segment intersects again at respectively, prove that .
(This problem was suggested by Zuming Feng.)

In triangle , points lie on sides , respectively. Let denote the circumcircles of triangles , respectively. Given the fact that segment intersects again at respectively, prove that .
(This problem was suggested by Zuming Feng.)

Assume that and intersect again at second point other than . If not, the degenerate case where and are tangent at can be dealt similarly. Because and are cyclic, we have and . Hence, we obtain
from which it follows that is cyclic. This means that , and meet at . Note that this is the statement of Miquel's theorem.
Because is inscribed in , we find . Because is inscribed in , we find . Because is inscribed in , we find . Thus, we have . In triangles and , we have and . Therefore, triangles and are similar to each other which in particular implies
In a similar manner, we can show that triangles and are similar to each other and that
Combining the last two equations yields the desired result.
We consider the configuration shown in the diagram above. We can adjust the proof easily for other configurations. In particular, our proof uses directed angles modulo .
Let line intersect again at (other than ). Because is cyclic, . Because is cyclic, . Hence triangles and are similar to each other. In particular, we conclude that
Likewise, if line intersect again at (other than ), we can show that triangles and are similar to each other and that
In the light of the second equalities in (12) and (13), it suffices to show that . On the other hand, the first equalities in (12) and (13) imply that lie on a line. But this line has only two intersections with with being one of them. Hence we must have , completing our proof.
We maintain the configuration and the notations of the second solution. Let denote the intersection of lines and . Because and are cyclic, we have and . Hence, triangles and are similar to each other. In particular, we have , implying that is cyclic. Therefore, lies on and . Because is cyclic, , hence and are corresponding points in the similar triangles and , from which the desired result follows.