sn=1/3+2/9+3/27+……+（n-1）/3（n-1）+n/3^n
3sn= 1/9 +2/27 + 3/81 … + （n-1）/ 3^n + n/3^n+1
sn-3sn= 1/3 + (1/9+1/27+1/81+……+1/3^n)-n/3^n+1
= 1/3+ (1-1/3^n-1)/1-1/3 -n/3^n+1
=1/3 +(3-1/3^n-2)/2- n/3^n+1
=11/6-(1/3^n-2)/2-n/3^n+1
sn=-11/12+(1/3^n-2)/4+(n/3^n+1)/2