设F（x,y,z）=sin z-xyz 则 F′ (X)=-yz F′(y)= -xz F′(z)= cosz-xy
z对x的谝导数等于 -yz/(cosz-xy)
z对y的谝导数等于 -xz/(cosz-xy)
dz=[-yz/(cosz-xy)]dx+[-xz/(cosz-xy)]dy