Creation of this applet was inspired by a [url=http://www.gogeometry.com/ArchBooLem13.htm]post[/url] from Antonio Gutierrez ([url=http://www.gogeometry.com/]GoGeometry[/url]). [br][br][b][color=#9900ff]How can we formally prove that the 2 purple segments are congruent? [/color][/b]