If \(\rm y = \sqrt{\cos x + \sqrt{\cos x+ \sqrt{\cos x + \sqrt {\cos x + ... \infty}}}}\), then \(\rm \frac{dy}{dx}\) is equal to
1
\(\rm \frac{\cos x}{1- 2y}\)
2
\(\rm \frac{\sin x}{1- 2y}\)
3
\(\rm \frac{-\sin x}{1- 2y}\)
4
\(\rm \frac{-\cos x}{1- 2y}\)