# Dynamic proof: angle sum in Triangle is 180 degrees

- Author:
- bruce willis

- Topic:
- Angles

Note the direction of the arrows at the start and at the end of the animation.
Note that on the way the proof also proves that the exterior angle equals the sum of the two opposing interior angles (second rotation of the line leaves it lying in the same direction as it would if it had initially rotated clockwise about the bottom left corner, through the exterior angle).