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

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Nov 8, 2016

#### Answer:

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 -->
$\cos \left(\frac{5 \pi}{6}\right) = - \cos \left(\frac{\pi}{6}\right) = - \frac{\sqrt{3}}{2}$

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