Hello PaymemoneyEdited version: second thoughts about my first answer.

In Britain, the inverse cosine function is almost always written as , whereas is widely used elsewhere. Since I posted my original answer, it occurred to me that I interpreted as the inverse cosine of , whereas the question might mean . Also, I am not clear how the domain of being restricted to is relevant.

So, unless someone can clarify what the question means, I'll keep quiet!

Grandad