inverse cos is the same as arc cos
so just type acos(number)=
The way to find the acos key is by first pressing "2nd" on top left. If you see 2 acos take the leftmost as I suppose the other is acosh.
I had the same problem and thought the fine print -1 on cos key meant to long press (browsing on a phone that seemed logical). Then I started looking for a shift key and find some arrows down to the right but no luck there either. After reading this answer I stopped looking for -1 symbols and found that "2nd" was the shift key. It would have helped if it was a different colour.
Or just typing on keyboard and ignoring the keys altogether. 🐒
Best regards, Another Guest