对于参数方程$y=f(t),x=g(t)$，求一阶导 $\frac{dy}{dx}=\frac{dy}{dt}\times\frac{dt}{dx}=\frac{f'(t)}{g'(t)}$ 那二阶导 $\frac{d^2y}{dx^2}=\frac{d^2y}{dt^2}\times\frac{dt^2}{dx^2}=\frac{f''(t)}{g'^2(t)}$ 为什么不对？