# How do I find the value of cos 5pi / 6?

Oct 25, 2015

Find exact value of $\cos \left(\frac{5 \pi}{6}\right)$

Ans: $\frac{\sqrt{3}}{2}$

#### Explanation:

On the trig unit circle,
$\cos \left(\frac{5 \pi}{6}\right) = \cos \left(- \frac{\pi}{6} + \pi\right) = - \cos \left(\frac{\pi}{6}\right)$
Trig Table of Special Arcs gives -->
$\cos \left(\frac{5 \pi}{6}\right) = - \cos \left(\frac{\pi}{6}\right) = - \frac{\sqrt{3}}{2}$