Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/916428095720185856]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br][b]You can move the vertices of the triangle anywhere you'd like.[br][/b][color=#1e84cc][b]You can also control the size of the blue angle by using the blue slider. [br][br][/b][/color][b][color=#ff00ff]How can we formally prove what this applet informally illustrates?[/color][/b]