Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/843453348896542721]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url]. [br][br]In the applet below, the [b][color=#1e84cc]blue circle[/color][/b] and [color=#38761d][b]green circle[/b][/color] are concentric circles. [br][br][b][color=#0000ff]How can you formally prove what this applet informally illustrates? [/color][/b]