Answer
$\dfrac{3\pi}{4}$
Work Step by Step
Suppose $\theta = \cos^{-1} (-\dfrac{\sqrt 2}{2})$
This gives: $\cos \theta= \dfrac{-\sqrt 2}{2}$
Now, $\theta = \cos^{-1} (-\dfrac{\sqrt 2}{2})=\dfrac{3\pi}{4}$
Therefore, $\cos^{-1} (\dfrac{-\sqrt 2}{2})=\dfrac{3\pi}{4}$
