x^n=（x-1+1)^n
=(x-1)^n+n(x-1)^(n-1)+.+n(x-1)^1+1
则(x^n-1)/(x-1)=(x-1)^(n-1)+n(x-1)^(n-2)+.n(n-1)(x-1)/2+n
n阶倒等于0