Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/880887535165210624]tweet[/url] from[url=https://twitter.com/gogeometry] Antonio Gutierrez[/url] (GoGeometry). [br][br]Feel free to move the vertices of this right triangle anywhere you'd like. [br][br][b][color=#666666]How can we formally prove the measure of the gray angle IS what it is? [/color][/b]