version：0.1
转载自： 传送门 知乎作者：湖心亭看雪
\[\begin{aligned}
\frac{d}{dx}(x^{2})
&= \frac{dx^{2}}{dx} \\
&= \frac{f(x+dx)-f(x)}{dx} \\
&= \frac{(x+dx)^{2}-x^{2}}{dx} \\
&= \frac{x^{2}+2xdx+dx^{2}-x^{2}}{dx} \\
&= \frac{2xdx+dx^{2}}{dx} \\
&= 2x+dx \\
&= 2x \text{}
\end{aligned}
\]