Creation of this applet was inspired by a [url=https://twitter.com/jldavilaa01/status/922028947352772609]tweet[/url] from [url=https://twitter.com/jldavilaa01]Jose Luis da Vila[/url]. [br][br][b][color=#ff00ff]What is the measure of the pink angle? [/color][/b][br][br][b]How can we formally prove this to be true? [/b]