设可微函数 $z=z(x, y)$ 满足 $x^2 \frac{\partial z}{\partial x}+y^2 \frac{\partial z}{\partial y}=2 z^2$, 又设 $u=x, v=\frac{1}{y}-\frac{1}{x}$,
$w=\frac{1}{z}-\frac{1}{x}$, 则对函数 $w=w(u, v)$, 偏导数 $\left.\frac{\partial w}{\partial u}\right|_{\substack{u=2 \\ v=1}}=$