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设 $z=u(x, y) e^{a x+b y}, \frac{\partial^2 u}{\partial x \partial y}=0$, 试确定 $a, b$ 使 $\frac{\partial^2 z}{\partial x \partial y}-\frac{\partial z}{\partial x}-\frac{\partial z}{\partial y}+z=0$
                        
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