Creation of this applet was inspired by a [url=http://www.gogeometry.com/problem/p400_triangle_angle_bisector_circumcircle.htm]post[/url] from [url=http://www.gogeometry.com/]Antonio Gutierrez[/url]. [br][br][b][color=#ff00ff]You can adjust the size of the angle with pink vertex using the pink slider. [br][br][/color][/b][color=#0000ff][b]How can we formally prove what this applet informally illustrates? [/b][/color]