Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/1000360032423333888]tweet[/url] posted by [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br]You can move the [b]LARGE WHITE VERTICES[/b] of the triangle [b]ANYWHERE[/b] you'd like.[br][color=#cc0000]You can also move the [b]LARGE RED POINT[/b] anywhere on the left side of this triangle. [/color][br][color=#666666]The [b]small gray slider[/b] controls the size of the (soon-to-appear) gray angle. [/color][br][br][b][color=#cc0000]How can we formally prove[/color] [color=#9900ff]what is dynamically illustrated here? [/color][/b]