y=f(g(x))
dy/dx=df(g(x))/d(g(x)) * d(g(x))/dx
如：
y=cos(x^2)
dy/dx=d(cos(x^2))/d(x^2) * d(x^2)/dx
dy/dx=-sin(x^2) * 2x
微分为：dy=-2xsin(x^2) dx