Answer
$\dfrac{\sqrt3}{2}$
Work Step by Step
Convert the angle measure to degrees to obtain:
$=-\frac{\pi}{6} \cdot \frac{180^o}{\pi} = -30^o$
Thus,
$\cos{(-\frac{\pi}{6})} = \cos{(-30^o)}$
$-30^o$ is co-terminal with $-30^o+360^o=330^o$.
$330^o$ is in Quadrant IV so its reference angle is $=360^o-330^o=30^o$.
Note that the cosine function is positive in Quadrant IV.
