1992 CMO Problem: Cocircular Orthocenters
Let , , , be distinct points on a circle (black) centered at . Let be the orthocenter of triangle and so on. You can show that the four orthocenters are cocircular and the circle (green) has the same radius as the original circle.
This Demonstration is based on a problem from the 2nd Section of the Chinese Math Olympic National Final in 1992.