e^z=xyze^z·∂z/∂x=yz+xy∂z/∂x→∂z/∂x=yz/(e^z-xy)∂²z/∂x∂y=[(z+y∂z/∂x)(e^z-xy)-yz·(e^z·∂z/∂x-x)]/(e^z-xy)²=[(z+y·yz/(e^z-xy))(e^z-xy)-yz·(e^z·yz/(e^z-xy)-x)]/(e^z-xy)²=[z·(e^z-xy)²+y²z(e^z-xy)-y²z²e^z+xyz(e^z-xy)]/(e^z-xy)³