Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/845644173139173376]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url]. [br][br]Note: The 2 circles are mutually tangent to each other. [br]Feel free to move any of the BIG POINTS anywhere you'd like! [br][br][b][color=#0000ff]How can you formally prove what this applet informally illustrates? [/color][/b]