an+1/an=1/3.{an}是等比数列,所以an=2(1/3)^n-1,
bn=an+n=2(1/3)^n-1+n
sn=2[1+1/3+(1/3)^2+----+(1/3)^n-1]-n+n(n+1)/2=3[1-(1/3)^n]+n(n-1)/2