Given a convex pentagon , let point be the intersection of lines and , let point be the intersection of lines and , and define points analogously. Furthermore, let point be the other intersection point of the circumcircle of triangle and the circumcircle of triangle , let point be the other intersection point of the circumcircle of triangle and the circumcircle of triangle , and define points analogously. Prove that lines are concurrent.
Solution
Perform an inversion with center and denote inverse point with prime.

Let , , , . Clearly , .
and are concyclic, so from Reim's theorem we get . Similarly, we can prove that .
Note that are concyclic, so from Reim's theorem we get , hence
which implies that are collinear and .
From Reim's theorem we get are concyclic, so lie on a circle , which implies that are concurrent at the radical center of , , and . Analogously, we can prove lies on and , so are concurrent.
Want a route through all this instead of an archive? The track
puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.