GoGeometry Action 74!

Creation of this applet was inspired by a [url=http://gogeometry.com/problem/problem019.htm]post[/url] from Antonio Gutierrez ([url=http://gogeometry.com/]GoGeometry[/url]). [br][br][color=#9900ff][b]How can we formally prove what this applet informally illustrates? [/b][/color]
Quick (Silent) Demo

Information: GoGeometry Action 74!