# How do you evaluate arccos(-1/2) without a calculator?

Dec 9, 2016

(2pi)/3;(4pi)/3

#### Explanation:

Use trig table for special arcs and unit circle -->
$\cos x = - \frac{1}{2}$--> There are 2 arcs that have the same cos value -->
$x = \frac{2 \pi}{3}$ and $x = \frac{4 \pi}{3}$
$x = \frac{2 \pi}{3} + 2 k \pi$
$x = \frac{4 \pi}{3} + 2 k \pi$