就是导数定义变形
题目错误，x0 应写为 1.lim[f(1-2h) - f(1-5h)]/h = lim[f(1-2h) - f(1) + f(1) -f(1-5h)]/h= lim[f(1-2h) - f(1)]/h - lim[f(1) - f(1-5h)]/h= -2lim[f(1-2h) - f(1)]/(-2h) - 5lim[f(1-5h) - f(1)]/(-5h)= -2f'(1) - 5f'(1) = -7f'(1) = 14
