GoGeometry Action 138!

Topic:
Square
Creation of this applet was inspired by a tweet posted by Antonio Gutierrez (GoGeometry). You can move the LARGE WHITE VERTICES of the triangle anywhere you'd like AT ANY TIME. You can also alter the size of the BLUE INTERIOR ANGLE by using the BLUE SLIDER. How can we formally prove the 3 (soon-to-appear) lines are ALWAYS CONCURRENT?

Quick (Silent) Demo