Creation of this applet was inspired by a [url=http://www.gogeometry.com/school-college/1/p1047-parallelogram-parallel-diagonal-concurrency-math.htm]problem posted[/url] by [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br]Feel free to move the [b]BIG WHITE POINTS[/b] around at any time. [br][b][color=#ff00ff]You can control the size of the pink angle using the pink slider. [/color][/b] [br][br][b][color=#1e84cc]How can we formally prove what is dynamically illustrated here? [/color][/b]