Creation of this applet was inspired by a problem posted in a [url=https://twitter.com/gogeometry/status/808442695807107072]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url]. [br][br][b]The white point on the circle on the left adjusts its radius. [/b] [br][color=#38761d][b]The green slider adjusts the radius of the circle on the right. [br][/b][/color][br][color=#666666][b]Feel free to drag the center of the circles anywhere you'd like! [br][/b][/color][br][color=#0000ff][i]How can you formally prove what this applet informally illustrates? [/i][/color]