bn=2n/2^nSn=b1+b2+b3+.+b(n-1)+bn=2·1/2+2x2·1/2²+2x3·1/2³+.+2n·1/2^n .①1/2·Sn= 2·1/2²+2x2·1/2³+.+2(n-1)·1/2^n +2n·1/2^(n+1).②①-②得：1/2·Sn=2(1/2+1/2²+1/2³+.+...