分别算
令F=e^z-xyzFz=e^z-xy, Fx=-yz, Fy=-xz,Zx=-Fx/Fz=yz/(xy-e^z), Zy=-Fy/Fz=xz/(xy-e^z), (按隐函数求导方法）dz=Zxdx+Zydy=yz/(xy-e^z) dx+xz/(xy-e^z) dy
