\(设函数u=\frac{1}{r},(r=\sqrt{x^2+y^2+z^2}\neq 0).则方程\frac{\partial^2u}{\partial x^2}+\frac{\partial^2u}{\partial y^2}+\frac{\partial^2u}{\partial z^2}=0成立.\)

\(设函数u=\frac{1}{r},(r=\sqrt{x^2+y^2+z^2}\neq 0).则方程\frac{\partial^2u}{\partial x^2}+\frac{\partial^2u}{\partial y^2}+\frac{\partial^2u}{\partial z^2}=0成立.\)

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