# Triangles Contain 180°

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

Try it yourself (drag the points):

geometry/images/triangle.js?mode=angles

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)

runs parallel to the base of the triangle.

Notice that:

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

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

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