## Trigonometry 7th Edition

$\dfrac{1}{2}$
$\dfrac{5\pi}{3}$ is an angle in Quadrant IV. The reference angle of an angle in Quadrant IV can be found using the formula $2\pi-\theta$. Thus, the reference angle of $\dfrac{5\pi}{3}$ is: $2\pi - \dfrac{5\pi}{3}=\dfrac{6\pi}{3}- \dfrac{5\pi}{3} = \dfrac{\pi}{3}$ $\dfrac{\pi}{3}$ is a special angle whose cosine value is $\dfrac{1}{2}$. $\dfrac{5\pi}{3}$ is in Quadrant IV where cosine is positive. Thus, $\cos{(\frac{5\pi}{3})}=\frac{1}{2}$