Creation of this applet was inspired by a [url=http://www.gogeometry.com/problem/p093_parallelogram_circle_elearning.htm]problem[/url] posted by [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br][b][color=#1e84cc]You can control the size of the interior angle with blue vertex using the blue sider. [/color][/b][br][b]You can move the LARGE POINTS anywhere you'd like at any time. [br][/b][br][color=#ff00ff][b]How can we formally prove what is dynamically illustrated here? [/b][/color]