e^x - 1 - x = e^x - e^0 - x = (e^ξ)x - x = (e^ξ - e^0)x = (e^ξ')ξx
其中,ξ在0和x之间,与x同号,ξ'在0和ξ之间,所以(e^ξ')ξx ≥ 0