If $y=\tan ^{-1}\left[\frac{4 \sin 2 x}{\cos 2 x-6 \sin ^2 x}\right]$, then $\frac{\mathrm{d} y}{\mathrm{~d} x}$ at $x=0$ is

If $x=a \sin 2 t(1+\cos 2 t), y=b \cos 2 t(1-\cos 2 t)$ then $\frac{\mathrm{d} y}{\mathrm{~d} x}$ is equal to

If $y=\sqrt{x+\sqrt{y+\sqrt{x+\sqrt{y+\ldots \ldots \ldots \ldots \ldots \ldots . \infty}}}}$, then $\frac{\mathrm{d} y}{\mathrm{~d} x}=$