Cut-the-Knot Action 15!

Creation of this applet was inspired by a [url=https://twitter.com/CutTheKnotMath/status/890361457719771136]tweet[/url] from [url=https://twitter.com/CutTheKnotMath]Alexander Bogomolny[/url]. [br][color=#ff00ff][b][br]How can we formally prove THIS to be true? [/b][/color]
Quick (Silent) Demo

Information: Cut-the-Knot Action 15!