Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/916036878540902400]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][color=#38761d][br]You can control the size of the interior angle with green vertex by using the green slider. [/color][br]Also, feel free to move any of the triangle's vertices anywhere you'd like at any time. [br][b][br]Why is the phenomenon (seen at the end) always true? [br]How can we formally prove this true? [/b]