# How do you prove the reduction identities Cos(x+pi)= -cos(x) using a unit circle diagram?

##### 2 Answers
Aug 19, 2015

I am not sure this little diagram is enough but...

#### Explanation:

Have a look:

Aug 20, 2015

Supplementary arcs: sin x and $\sin \left(x + \pi\right)$

#### Explanation:

The trig unit circle shows the relationship between the supplementary arcs:

$\sin x = \sin \left(\pi - x\right)$

$\sin x = - \sin \left(\pi + x\right)$