Creation of this applet was inspired by a [url=http://www.gogeometry.com/school-college/4/p1324-quadrilateral-isosceles-45-degree-angle.htm]post from Antonio Gutierrez[/url] ([url=http://gogeometry.com/]GoGeometry[/url]). [br][br][b][color=#0000ff]How can you formally prove the phenomena dynamically illustrated here? [/color][/b]