cos((3π/2) - ϴ)
\(cos((3π/2) - \theta)\\ =\text{change to degrees}\\ =cos(270-\theta)\\ =cos(180+(90-\theta))\\ =-cos(90-\theta)\\ =-sin(\theta)\)
I think Max is right in that not all this working is really needed.
But it is easier for me to think through this way :)