Answer
$cos^{-1}(-\frac{\sqrt{2}}{2}) = \frac{3\pi}{4}$
Work Step by Step
The domain of $cos^{-1}(x)$ is $[-1, 1]$ and the range is $[0, \pi]$. $cos(\frac{3\pi}{4}) = -\frac{\sqrt{2}}{2}$, so $cos^{-1}(-\frac{\sqrt{2}}{2}) = \frac{3\pi}{4}$.
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