Let be a trapezium, , and let and be points on the sides and , respectively. The circumcircle of the triangle meets the line again at , and the circumcircle of the triangle meets the line again at . Prove that the lines , , and are concurrent.
Solution
First solution. Let the line meet the circle again at . Then
so and are parallel. Similarly,
so and are parallel.
Thus, the corresponding sides of the triangles and are parallel. Since the vectors and are counter-directed, these triangles are homothetical under a homothety of negative ratio.
The centre of this homothety lies on the lines , , and , so the three are concurrent.

Second solution. Let the lines and meet at , and let the lines and meet at . Since
the points , , , and are concyclic. Thus , showing that is parallel to both and .
This shows the triangles and in perspective from the ideal point of the three parallel lines, so the lines , and are concurrent, by Desargues' theorem; since the segments and have a non-empty intersection, the point of concurrency is not ideal.