z = x + ysin(z) 两边对x求偏导∂z/∂x = 1 + ycos(z) ∂z/∂x∂z/∂x {1 - ycos(z)} = 1解出：∂z/∂x ＝ 1/{1 - ycos(z)}
两边取全微分即可dz=dx+sinzdy+ycoszdz所以dz=dx /(1-ycosz)+sinzdy /(1-ycosz)所以dz/dz=1/(1-ycosz)
