# 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°**