# How do you evaluate cos^-1(-1)?

$\left(2 k + 1\right) \pi , k = 0 , \pm 1 , \pm 2 , \pm 3. . .$
If $a = {\cos}^{- 1} \left(- 1\right) , \cos a = - 1$
If the range of a is specified as , say, $\left[0 , 2 \pi\right] , a = \pi$.
The general solution is a = $\left(2 k + 1\right) \pi , k = 0 , \pm 1 , \pm 2 , \pm 3. . .$