How do I find the inverse of a $\cos^2 \theta$?

Question

This was originally a physics question, but the math is what is throwing my brain into loops.

Basically, I need to find $\theta$: $$ \frac{7}{8}= \cos^2(\theta) $$

Answer 1

Hint: $\displaystyle \cos^2 \theta = \frac{7}{8}$ simply means that $\displaystyle \cos \theta = \pm\sqrt{\frac{7}{8}}$. Can you take it from there? The answer will not be a multiple of $\pi$ (assuming you're using radians).

Answer 2

$\frac{7}{8}=\cos^2\theta\\\implies \pm\sqrt{\frac{7}{8}}=\cos\theta\\\implies\theta=\arccos\sqrt{\left(\frac{7}{8}\right)}$