Creation of this applet was inspired by a [url=http://gogeometry.blogspot.com/2015/09/geometry-problem-1149-isosceles.html]problem[/url] posted by [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br][b]LARGE POINTS[/b] are moveable at any time.[br][color=#ff00ff][b]You can also control the size of the angles with pink vertices using the pink slider. [/b][/color][br][br][b]How can we formally prove the phenomenon dynamically illustrated here? [/b][br][br]