version：0.1
转载自： 传送门 知乎作者：湖心亭看雪
[egin{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 ext{}
end{aligned}
]