# GoGeometry Action 189!

- Author:
- Tim Brzezinski

Creation of this applet was inspired by this problem posted by Antonio Gutierrez (GoGeometry).
You can change the size of the

**interior angle in the lower left corner**by using the**smaller slider**. Feel free to place any of the triangle's vertices where you'd like at any time.**How can we formally prove what is dynamically illustrated here?**