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Butterfly Theorem

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Let D be the midpoint of chord BC of a circle, through which two other chords EH and FG are drawn. EG and HF intersect chord BC at I and J, respectively. The Butterfly Theorem states that D is the midpoint of IJ.

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Proof

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Link to a good proof :

http://agutie.homestead.com/FiLEs/GeometryButterfly.html

See also

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