Trigonometry (11th Edition) Clone

$\dfrac{\sqrt3}{2}$
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.