Creation of this resource was inspired by a [url=https://twitter.com/gogeometry/status/1014703639070470144]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] (GoGeometry). [br][br][b]LARGE POINTS[/b] are moveable at any time.[br][br][color=#1e84cc]The [/color][b][color=#1e84cc]LIGHT BLUE SLIDER[/color] [/b][color=#1e84cc]controls the size of the triangle's [/color][b][color=#1e84cc]interior angle with blue vertex[/color][/b].[br][br][color=#ff00ff][b]How can we formally prove what is dynamically illustrated here? [/b][/color]