(6) x/z = ln(z/y) = lnz - lny两边分别对 x， y 求偏导数，得(z-xz')/z^2 = z'/z-xz'/z^2 = z'/z - 1/y∂z/∂x = z' = z/(x+z)∂z/∂y = z' = z^2/[y(x+z)] z'' = [z'(x+z)-z(1+z']/(x+z)^2= -z^2/(x+z)^3z'' = [2zz'y(x+z) - z^2(x+z+yz')]/[y^2(x+z)^2]= -x^2z^2/[y^2(x+z)^3}
