Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/866706149508345857]tweet[/url] from [url=https://twitter.com/gogeometry/]Antonio Gutierrez[/url]. [br][br][b][color=#1e84cc]What is the measure of the blue angle? [br][/color][color=#9900ff]How can we formally prove this true? [/color][/b]