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.


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This Demonstration is based on a problem from the 2nd Section of the Chinese Math Olympic National Final in 1992.
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