Now that I've got the first value, how do I find the second? All I know is that the CAST rule may be involved, but I don't know how to apply it in this situation. Help on this would be greatly appreciated.
The corresponding identity for sin(x) is sin(x) = -sin(-x) because sin(x) is odd.
It is also true that sin(x) = sin(180-x) but this more closely corresponds with cos(x) = -cos(180-x). See here.