**Contents**

### The 180^{o} Rule

- For any given
**triangle**, the**sum of the inside angles**is equal to**180**:^{o}

A+B+C=180^{\circ}

Where A, B and C are the inside angles of a triangle.

Example

We wish to find the unknown angle, X, in the above triangle. As the other angles are known, we can use the 180^{o} rule. First, we substitute in the known values:

X+35+45=180

Now we solve for the unknown angle:

X+80 &=180

X &=100^{\circ}