Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/917068349611347968]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url]. [br][br][b]You can move the LARGE WHITE VERTICES of the triangle anywhere you'd like.[/b][br]In addition, feel free to move the [b][color=#9900ff]LIGHT PURPLE POINT[/color][/b] anywhere along the bottom side of the triangle. [br][b][color=#ff00ff]You can also alter the size of the pink angle using the pink slider. [/color][/b][br][b][color=#1e84cc][br]How can we formally prove what is dynamically illustrated here?[/color][/b] [br][br]