Square + Rhombus = Constant Surprise! (GoGeometry Action 71)

Creation of this applet was inspired by a [url=https://twitter.com/gogeometry/status/880023738410033152]tweet[/url] from [url=https://twitter.com/gogeometry]Antonio Gutierrez[/url] ([url=http://gogeometry.com/]GoGeometry[/url]). [br][br][b][color=#1e84cc]You can adjust the size of the blue angle (soon to appear) using the blue slider. [/color][/b] [br][b]You can also move any of the 2 LARGE WHITE POINTS anywhere you'd like.[/b] [br][br][b][color=#ff00ff]What is the measure of the pink angle?[br][/color][color=#ff00ff][br][/color][color=#ff00ff]How can we formally prove this is ALWAYS TRUE [br][/color][color=#1e84cc](regardless of what the original rhombus looks like?)[/color][/b]
Quick (Silent) Demo

Information: Square + Rhombus = Constant Surprise! (GoGeometry Action 71)