已知：u=f(x,y)u=f(x,y)u=f(x,y)有二阶连续偏导数，计算∂2u∂x2−∂2u∂y2\frac{\partial^2 u}{\partial x^2}-\frac{\partial^2 u}{\partial y^2}∂x2∂2u​−∂y2∂2u​在新的坐标系下对应的表达式。 {s=x+yt=x−y\left\{ \begin{aligned} s & = & x+y \\ t & = & x-y \\ \end{aligned} \right. {st​==​x+yx−y​