Answer
$\cos{\frac{2\pi}{3}}= -0.5$
Work Step by Step
The angle $\dfrac{2\pi}{3}$ is in Quadrant II.
Its reference angle is $\dfrac{\pi}{3}$.
Note that $\dfrac{2\pi}{3}$ is a special angle and that $\cos{(\frac{\pi}{3})}=0.5$.
The trigonometric function values of an angle and its reference angle are either the same or differ only in signs.
Since $\dfrac{2\pi}{3}$ terminates in Quadrant II, then its cosine is negative.
Therefore,
$\cos{\frac{2\pi}{3}}= -0.5$.
