设 z=z(x,y) 满足方程 -zy=0 用变量替-|||-换 =dfrac (s+n)(2) =dfrac (z-n)(2) 得到的方程为 () .-|||-A dfrac (partial z)(partial f)=0.-|||-B dfrac (partial z)(partial n)=0-|||-C dfrac (partial z)(partial z)+dfrac (partial z)(partial u)=0.-|||-D dfrac (partial z)(partial z)-dfrac (partial z)(partial u)=0