GoGeometry Action 75!

Creation of this applet was inspired by a [url=http://www.gogeometry.com/school-college/1/p1042-triangle-isosceles-angle-congruence-parallelogram-math.htm]post[/url] from Antonio Gutierrez ([url=http://www.gogeometry.com/]GoGeometry[/url].) [br][br][b][color=#1e84cc]You can change the size of the angle (of the triangle) with blue vertex by adjusting the blue slider.[/color][/b][br][color=#ff00ff][b]You can change the size of the (soon-to-appear) pink angle by adjusting the pink slider. [br][/b][/color][br][b][color=#0000ff]How can we formally prove what this applet informally illustrates?[/color][/b] [br]
Quick (Silent) Demo

Information: GoGeometry Action 75!