x+y+z+xyz = 0,两边对 x 求偏导，得 1 + z' + yz + xzz' = 0,z' = -(1+yz)/(1+xz)f = e^x y z^2f' = ye^x z^2 + ye^x 2zz' = ye^x z^2 - 2yze^x (1+yz)/(1+xz)
