question

cos theta is a ratio, it is always between -1 and +1

inverse cos is an angle.    Inverse cos is the same as arc cos.

In the trianlge illustrated:

\(\boxed{cos \theta = \frac{adj}{hyp}\\~\\ cos^{-1}\left( \frac{adj}{hyp}\right)=\theta\\}\\~\\ also\\~\\ acos\left( \frac{adj}{hyp}\right)=cos^{-1}\left( \frac{adj}{hyp}\right)=\theta\\~\\\)