Creation of this applet was inspired by a [url=https://twitter.com/CutTheKnotMath/status/846156292263686144]tweet[/url] from [url=https://twitter.com/CutTheKnotMath]Alexander Bogomolny[/url]. [br][b][color=#0000ff][br]How can you formally prove what this applet informally illustrates? [/color][/b]