例题
已知微分方程
$$
\frac{\partial^2 u}{\partial x^2}+\frac{\partial^2 u}{\partial y^2}=0
$$
有形如 $u=\varphi(\frac{y}{x})$ 的解，试求此解．
已知微分方程
$$
\frac{\partial^2 u}{\partial x^2}+\frac{\partial^2 u}{\partial y^2}=0
$$
有形如 $u=\varphi(\frac{y}{x})$ 的解，试求此解．
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