lnz=yln(1+xy)Z'x/z=y^2/(1+xy)---> Z'x=zy^2/(1+xy)Z'y/y=ln(1+xy)+xy/(1+xy)---> Z'y=zln(1+xy)+xyz/(1+xy)dz=Z'xdx+Z'ydy=zy^2/(1+xy)dx+[zln(1+xy)+xyz/(1+xy)]dy