Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/848224241044652032]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url]. [br][br][b]Feel free to move the triangle's vertices anywhere you'd like. [/b][br][b][color=#6aa84f]You can adjust the size of the light green angle by using the light green slider. [/color][/b][br][br][b][color=#0000ff]How can you formally prove what this applet informally illustrates? [/color][/b]