Butterfly Theorem Proof: Circle, Chord, Midpoint,
Radius, Inscribed Angle, SAS Similarity, Cyclic Quadrilateral,
ASA Congruence. Level: High School, College.
HTML5 Animation for iPad and Nexus

The figure above shows a circle O and a chord AB with midpoint C. The chords DE and FG passing through C intersect AB at M and N, respectively. Prove that C is the midpoint of MN.