# GoGeometry Action 91!

- Author:
- Tim Brzezinski

- Topic:
- Congruence, Equilateral Triangles

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

**interior angle with purple vertex**by using the**purple 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?**