Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/583075667362897920]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry.) . [br][br]Feel free to move the white vertices of the right triangle anywhere you'd like. [br][br][b][color=#9900ff]How can we formally prove what this applet informally illustrates? [/color][/b]