## Precalculus (6th Edition)

$y=\displaystyle \frac{3\pi}{4}$
By definition of arccos, y is the number from $[0, \pi]$ such that $\displaystyle \cos y=-\frac{\sqrt{2}}{2}.$ Using the unit circle, $\displaystyle \cos\frac{3\pi}{4}=-\frac{\sqrt{2}}{2}$, and $\displaystyle \frac{3\pi}{4}\in[0, \pi] ,$ so $y=\displaystyle \frac{3\pi}{4}$