$$f:R->R$$
$$\frac{df}{dx}=\frac{df}{du}\frac{du}{dx}$$
$$x\inR\Rightarrow\frac{d}{dx}=0$$
$$x=u? ??$$
$$\Rightarrow\frac{df}{dx}=\frac{d}{dx}f\frac{d}{dx}$$
$$(\frac{d}{dx}f)(0)$$
$$\qed\frac{d}{dx}f(x) = 0$$