In the applet below, what is the measure of the BIG PINK ANGLE formed INSIDE the quadrilateral? How can you prove this true? (Feel free to drag the white point(s) anywhere you'd like.)
For more information, see this [url=https://twitter.com/gogeometry/status/843089288048840704]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url].