Creation of this applet was inspired by a [url=https://twitter.com/CutTheKnotMath/status/854791751198011393]tweet[/url] from [url=https://twitter.com/CutTheKnotMath]Alexander Bogomolny[/url]. [br][br][color=#0000ff][b]How can you formally prove what this applet informally illustrates? [/b][/color][br]