z' = f'<1> + yf'<2>注意 f'<1>, f'<2> 仍是 x，y 的复合函数, 得z'' = ∂(f'<1> + yf'<2>)/∂y = ∂(f'<1>)/∂y + f'<2> + y∂(f'<2>)/∂y = f''<11>∂u/∂y + f''<12>∂v/∂y + f'<2> + y[f'<21>∂u/∂y + f''<22>∂v/∂y]= f''<11> + xf''<12> + f'<2> + y[f'<21> + xf''<22>]= f'<2> + f''<11> + (x+y)f''<12> + xyf''<22>
