The Butterfly Theorem

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Let HE be a chord of a circle and CF and DG be two other chords passing through the midpoint P of HE. If CG and DF intersect HE, denote the points of intersection by A and B. When CG and DF intersect HE, P is the midpoint of AB.


Drag any of the magenta points on the circle to modify the diagram.


Contributed by: Jay Warendorff (March 2011)
Open content licensed under CC BY-NC-SA




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