例如：等比数列1/2,1/4,1/8,...1/2^n
Sn=1/2+1/4+1/8+...+1/2^n
=(1-1/2)+(1/2-1/4)+(1/4-1/8)+...+(1/2^(n-1)-1/2^n)
=1-1/2^n