an= n+ 2^(1-n)
｛n｝等差数列,首项1,公差1
｛2^(1-n)｝是等比数列,首项 1,公比为 2^(-1)
Sn=n(n+1)/2 + [1-2^(-n) ] / (1- 1/2) =n(n+1)/2 + 2 - 1/2^(n-1)