Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/871192531513552896]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url]. [br][br]Feel free to move any of the triangle's vertices anywhere you'd like. [br][br][b][color=#0000ff]How can we formally prove what is informally illustrated here? [/color][/b]