Given cosθ=-3/5 and π < θ < 3π/2, use an appropriate trigonometric identity to evaluate exactly.

1) cos θ/2

I got :

√(1+cosθ/2) = √(1+(-3/5)/2) = √((2/5)/2) = √(5)/5

I realize that they are referring to quadrant 3, I'm not really sure why that is relevant in the question. Could someone please explain that to me? And also if I have the right answer?

Thank you very much!