## Precalculus (6th Edition)

$y=\displaystyle \frac{2\pi}{3}$
By definition of arccos, y is the number from $[0, \pi]$ such that $\displaystyle \cos y=-\frac{1}{2}.$ Using the unit circle, we find that $\displaystyle \frac{2\pi}{3}\in[0, \pi]$and $\displaystyle \cos\frac{2\pi}{3}=-\frac{1}{2}.$ $y=\displaystyle \frac{2\pi}{3}$