Answer
$\dfrac{5\pi}{6}$
Work Step by Step
Suppose $\theta = \cos^{-1} (-\dfrac{\sqrt 3}{2})$
This gives: $\cos \theta=- \dfrac{\sqrt 3}{2}$
Now, $\theta = \cos^{-1} (-\dfrac{\sqrt 3}{2})=\dfrac{5\pi}{6}$
Therefore, $\cos^{-1} (-\dfrac{\sqrt 3}{2})=\dfrac{5\pi}{6}$
