答：
记Sn=1/3+4/9+7/27+…+(3n-5)/3^(n-1)+(3n-2)/3^n
则3Sn=1+4/3+7/9+…+(3n-2)/3^(n-1)
3Sn-Sn=2Sn
=1+(4/3-1/3)+(7/9-4/9)+…+(3n-2-(3n-5))/3^(n-1)-(3n-2)/3^n
=1+(1+1/3+1/9+…+1/3^(n-2))-(3n-2)/3^n
=1+[1-1/3^(n-1)]/(1-1/3)-(3n-2)/3^n
=1+3/2-1/(2*3^(n-2))-(3n-2)/3^n
=5/2-(6n+5)/(2*3^n)
所以Sn=5/4-(6n+5)/(4*3^n)