## Precalculus (10th Edition)

$\dfrac{\sqrt3}{2}$
We know that $\sin$, $\csc$, $\tan$ are odd functions, which means $f(-\theta)=-f(\theta).$ We also know that $\cos$, $\sec$, are even functions, which means $f(-\theta)=f(\theta).$ Therefore, $\cos(-30^{o})=\cos(30^{o})=\dfrac{\sqrt3}{2}$