Math trigonometry

Question

of x is the measurement of an acute angle whose cosine is x?

This is the definition of a word, and I need to know what the word is.

Answer 1

the principal range of f(x)=cos^-1(x) is a positive acute angle if 0<x<1

If x is -1<x≤1 then f(x)=cos^-1(x) has principal values range of angles from π^- to 0

Answer 2

Arccos(x) or cos^-1(x)