科普视频
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y对x的二阶导数：$\frac{d^2y}{dx^2}$
$ d^2f=d(df)=(f'(x)dx)^{'}dx=f^{''}(x)dxdx=f^{''}(x)dx^2 $ 其中^2=dxdx$，注意^2\neq d(x^2)=2xdx$