等比数列求和
Sn=a1(1-q^n)/(1-q)=(a1-anq)/(1-q)
x^0+x^1+x^2+...x^n+…是 x的等比数列
x^0+x^1+x^2+...x^n+…=（a1-anq)/(1-q)
=（1-(x^n)x)/1-x 因为-1