Question

Use the unit circle to evaluate the followingHow do I solve this problem?

Answer 1

Notice that:

$\cos (\frac{4\pi}{3})=\cos (\pi+\frac{\pi}{3})\text{.}$

Recall that:

$\cos (\pi+x)=-\cos (x)\text{.}$

Therefore:

$\cos (\frac{4\pi}{3})=-\cos (\frac{\pi}{3})=-\frac{1}{2}\text{.}$

Answer:

$cos(\frac{4\pi}{3})=-\frac{1}{2}\text{.}$