Brokard's theorem

Brokard's theorem is a theorem in projective geometry.

[1] It is commonly used in olympiad mathematics.

The points A, B, C, and D lie in this order on a circle

Lines AC and BD intersect at P, AB and DC intersect at Q, and AD and BC intersect at R. Then O is the orthocenter of

[2]This geometry-related article is a stub.