# Triangles Contain 180°

## In a triangle, the three interior angles always add to 180°:A + B + C = 180° |

Try it yourself (drag the points):

We can use that fact to find a missing angle in a triangle:

### Example: Find the Missing Angle "C"

Start With:A + B + C = 180°

Fill in what we know:38° + 85° + C = 180°

Rearrange C = 180° − 38° − 85°

Calculate:C = 57°

## Proof

This is a proof that the angles in a triangle equal 180°:

The top line (that touches the top of the triangle) is

running parallel to the base of the triangle.

So:

- angles A are the same
- angles B are the same

And you can easily see that A + C + B does a **complete rotation** from one side of the straight line to the other, or **180°**