Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/910098269291843585]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br][b]Feel free to move the vertices of this cyclic quadrilateral at any time. [/b] [br][br][color=#0000ff][b]How can we formally prove the phenomena dynamically illustrated here?[/b][/color]