e^(-xy)-2z+e^z=0-ye^(-xy)-2z'(x)+e^z z'(x)=0z'(x)=ye^(-xy)/(e^z-2)-xe^(-xy)-2z'(y)+e^z z'(y)=0z'(y)=xe^(-xy)/(e^z-2)dz=ye^(-xy)/(e^z-2) *dx + xe^(-xy)/(e^z-2) *dy=[ye^(-xy)dx+xe^(-xy)dy]/(e^z-2)