Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/854072460169367552]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url]. [br][br][b][color=#ff00ff]What is the measure of the pink angle? [/color][/b] [br][b]How can you formally prove this to be true?[/b] [br]