Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/895367614628757504]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br][b][color=#9900ff]You can use the purple slider to control the size of the interior angle with purple vertex. [/color][/b][br]Feel free to move the vertices of the triangle anywhere you'd like at any time. [br][br][color=#0000ff][b]How can we formally prove the phenomena witnessed at the end of this action? [/b][/color]