[color=#000000]Creation of this applet was inspired by a [url=https://twitter.com/solvemymaths/status/771904150447722496]Twitter tweet[/url] from [url=https://twitter.com/solvemymaths]Solve My Maths[/url]. [br][br]The following illustration dynamically illustrates [/color][b]Van Aubel's Theorem.[/b][color=#000000][br]This theorem holds true for ALL QUADRILATERALS (convex and concave). [br]So, feel free to put the [/color][color=#ff7700][b]BIG ORANGE VERTICES[/b][/color][color=#000000] anywhere you'd like (prior to sliding the slider). [br][br]How can you formally prove what this applet informally illustrates? [/color]