利用e^x 的幂级数展开：e^x = 1 + x + x^2/2! + x^3/3! + x^4/4! +...+ x^n/n!+...所以(e^x - 1) / x = x/2! + x^2/3! + ... + x^(n-1)/n! + ...两边对x求导得d((e^x-1)/x)/dx = 1/2! + 2x/3! + ... + (n-1)x^(n-2...