1/2+1/4+1/8+1/16+.......+1/128
1/2+1/4+1/8+1/16+.......+1/128
1/2 1/4 1/8 1/16为等比数列，公比q=1/2 1/128为1/2^7 即为1/2^n（n=1.2.3----7) a1=1/2 根据公式Sn=a1(1-q^n)/1-q 得 Sn=1/2(1-1/2^7)/(1-1/2) =1/2(1-1/128)/(1/2) =127/128