Creation of this applet was inspired by a [url=http://www.gogeometry.com/school-college/2/p1150-circle-secant-90-degrees-perpendicular-congruence.htm]problem[/url] posted by [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br]You can move any of the [b]4 LARGE POINTS[/b] anywhere you'd like! [br][color=#0000ff][br][b]How can we formally prove the phenomenon dynamically illustrated here? [/b][/color]