# How do you evaluate cos^-1(1/sqrt2)?

${\cos}^{- 1} \left(\frac{1}{\sqrt{2}}\right) = \pm \frac{\pi}{4}$
As $\cos \left(\pm \frac{\pi}{4}\right) = \frac{1}{\sqrt{2}}$
and $\cos$ ratio is positive in first and fourth quadrant
${\cos}^{- 1} \left(\frac{1}{\sqrt{2}}\right) = \pm \frac{\pi}{4}$