这样来的：Sn=a1+a1q+a1q²+......+a1q^(n-1)qSn= a1q+a1q²+......+a1q^(n-1)+a1q^n两式对应相减得：Sn-qSn=a1-a1q^n得Sn=a1(1-q^n)/(1-q)
