# How do you find the value for cos^ -1 (- sqrt 2/2)?

It is $\frac{3 \pi}{4}$
Hence $\cos \left(\frac{3 \pi}{4}\right) = - \frac{\sqrt{2}}{2}$
$\arccos \left(- \frac{\sqrt{2}}{2}\right) = \arccos \left(\cos \left(3 \frac{\pi}{4}\right)\right) = \frac{3 \pi}{4}$