x=f(xz,yz)两边对x求导：
1=f1(z+x∂z/∂x)+f2(y∂z/∂x) ∂z/∂x=(1-zf1)/(xf1+yf2)
x=f(xz,yz)两边对y求导：
0=f1(x∂z/∂y)+f2(z+y∂z/∂y) ∂z/∂y=(-zf2)/(xf1+yf2)
dz=[(1-zf1)/(xf1+yf2)]dx+[(-zf2)/(xf1+yf2)]dy