When is cos(x) = 0 or when is cos(x) = -1
What are the values of x when this values are satisfied
For x= 0 This happens when x= 90degree or x = - 90 degrees
And for cos(x)= -1 this happens when x= 180degree or x= -180 degree
This is done using the property that
cos(-x) =cos(x)
ie; if the answer is 90 degree than answer is also -90 degree as this value also lies in the domain
Read this
http://en.wikipedia.org/wiki/Trigonometric_function