Creation of this applet was inspired by a [url=https://twitter.com/CutTheKnotMath/status/850762247597166592]tweet[/url] from [url=https://twitter.com/CutTheKnotMath]Alexander Bogomolny[/url]. [br][br]Note the rectangles shown are similar to each other. [br][br][color=#0000ff][b]How can you formally prove what this applet informally illustrates? [/b][/color]