Creation of this applet was inspired by [url=http://www.gogeometry.com/problem/p241_triangle_equilateral_congruence_sas.htm]this problem[/url] posted by [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br]You can change the size of the [color=#9900ff][b]interior angle in the lower left corner[/b][/color] by using the [color=#9900ff][b]smaller slider[/b]. [/color] [br]Feel free to place any of the triangle's vertices where you'd like at any time. [br][color=#0000ff][br][b]How can we formally prove what is dynamically illustrated here?[/b][/color]