对x求导：3z^2 z'-3yz-3xyz'=0得：z'=yz/(z^2-xy)再对x求导：z"=y[z'(z^2-xy)-z(2zz')]/(z^2-xy)^2=-yz'(z^2+xy)/(z^2-xy)^2=-y^2 z/(z^2-xy)^3
