通项an=n/(2^n)=n×2^(-n)
∴ Sn=1×2^(-1)+2×2^(-2)+3×2^(-3)+……+n×2^(-n)①
∴2Sn=1×2^0+2×2^(-1)+3×2^(-2)+……+n×2^(1-n) ②
②-①得 Sn=[2^0+2^(-1)+2^(-2)+……+2^(1-n)]-n×2^(-n)
=2-2^(1-n)-n×2^(-n)