Copia de GoGeometry Action 166!

Creation of this resource was inspired by a problem posted by Antonio Gutierrez. The LARGE VERTICES of the triangle below can be moved anywhere at any time. The light green slider controls the measure of the interior angle with green vertex. Questions to consider: 1) How would you describe the phenomenon you see here in your own words? 2) How can we formally prove this phenomenon holds true for ANY TRIANGLE?