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© Copyright 2003, Jim Loy
The butterfly theorem:
We have a circle with chord AB with midpoint M. We draw two more chords CD and EF, both going through M. Draw CF intersecting AB at G, and DE intersecting AB at H. Then M is the midpoint of GH.
G and H can be outside the circle. Move the large red points in the diagram:
The above Java interactive demonstration was created with Cinderella (a geometry program).
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