If \( y(x) = \cot^{-1}\!\left(\sqrt{\frac{\sqrt{1 + \sin x} + \sqrt{1 - \sin x}}{\sqrt{1 + \sin x} - \sqrt{1 - \sin x}}}\right),\ x \in \left( \frac{\pi}{2}, \pi \right) \) then \( \frac{dy}{dx} \) at \( x = \frac{5\pi}{6} \) is:
\( \frac{1}{2} \)
\( -\frac{1}{2} \)
\( 0 \)
\( -1 \)