Creation of this applet was inspired by a [url=http://www.gogeometry.com/problem/p763-parallelogram-diagonal-parallel-congruence-plane-geometry.htm]post[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br][b][color=#1e84cc]You can change the measure of the blue angle (soon to form) using the blue slider.[/color][/b][br]Feel free to move the [b][color=#ff7700]BIG ORANGE POINT (soon to appear)[/color][/b] at any time as well. [br]The [b]LARGE WHITE POINTS [/b]are also moveable. [br][br][br][b][color=#9900ff]How can we formally prove what is informally illustrated here? [/color][/b][br]