设函数 $f(u, v)$ 具有二阶连续偏导数，函数 $g(x, y)=x^2+y^2-f(x+y, x y)$ ．求 $\frac{\partial^2 g}{\partial x^2}-2 \frac{\partial^2 g}{\partial x \partial y}$ $+\frac{\partial^2 g}{\partial y^2}$ ．
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