Creation of this applet was inspired by a [url=http://www.gogeometry.com/ArchBooLem15.htm]post[/url] from Antonio Gutierrez ([url=http://www.gogeometry.com/]GoGeometry[/url]). [br][br][b][color=#0000ff]How can we formally prove what this applet informally illustrates? [/color][/b]