Suppose $\frac{\pi}{2}<\theta<\frac{3 \pi}{2}$ And $\sin \theta=\frac{1}{2}$ Evaluate Cos

Suppose $\frac{\pi}{2}<\theta<\frac{3 \pi}{2}$ and $\sin \theta=\frac{1}{2}$ . Evaluate cos $\theta$ . Give the exact answer.