If it is Cos -1 (0) and cos is capitalized, that means it is a function and you apply restrictions. Based on the restrictions you get one possible value (I think the value is pi/2)
However, if the problem is cos -1(0) with no capitalization, it is a relation meaning values are infinite because of repetitions. The answers would be pi/2 and 3pi/2 and both would be added/subtracted by 2n(pi).
