Creation of this applet was inspired by a [url=http://www.gogeometry.com/problem/p408_cyclic_quadrilateral_perpendicular_parallelogram_congruence.htm]post[/url] from Antonio Gutierrez ([url=http://www.gogeometry.com/]GoGeometry[/url]). [br][br]Feel free to move any of the BIG POINTS anywhere you'd like. [br][br][color=#0000ff][b]How can we formally prove what this applet informally illustrates? [/b][/color]