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GoGeometry Action 29!

Creation of this applet was inspired by a tweet from Antonio Gutierrez. The phenomena you witness holds true for ALL TRIANGLES. Feel free to move any 1 (or more) of the triangle's vertices (BIG WHITE POINTS) around any time. You can also change the size of the blue angle by using the blue slider. What is the measure of the pink angle? How do you know this? How can you FORMALLY PROVE what this applet informally illustrates?

Quick (Silent) Demo