Cut-The-Knot Action (4)!

Creation of this applet was inspired by a [url=https://twitter.com/CutTheKnotMath/status/839535085967847424]tweet[/url] from [url=https://twitter.com/CutTheKnotMath]Alexander Bogomolny[/url] ([url=http://www.cut-the-knot.org/]cut-the-knot.org)[/url]. [br][br][b][color=#0000ff]How can you formally prove what this applet informally illustrates? [/color][/b]
Quick (Silent) Demo

Information: Cut-The-Knot Action (4)!