$\lim\limits_{x \to 0} \dfrac{\ln(\cos x)}{x^2}$.

已知函数$y = \sin2x-\sqrt{2x + 1}$，求$dy$.

已知$y = y(x)$由方程$y^{3}+y^{2}=2x$所确定，求$\dfrac{dy}{dx}$.