F=x^2 +y^2 +z^2-xyz,Fx=2x-yz, Fy=2y-xz, Fz=2z-xy,∂z/∂x=-Fx/fz=-(2x-yz)/(2z-xy),∂z/∂y=-Fy/fz=-(2y-xz)/(2z-xy),dz=-(2x-yz)/(2z-xy)•dx-(2y-xz)/(2z-xy)•dy.
