Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/854124746056884224]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url]. [br][br][b]The phenomena you witness holds true for ALL TRIANGLES. [/b][br][br]Feel free to move any 1 (or more) of the triangle's vertices (BIG WHITE POINTS) around any time. [br][color=#1e84cc]You can also change the size of the blue angle by using the blue slider. [/color] [br][color=#ff00ff][br]What is the measure of the pink angle? How do you know this? [/color][br][br][b][color=#0000ff]How can you FORMALLY PROVE what this applet informally illustrates? [/color][/b]