是求前n项和Sn吗?
Sn=a1+a2+a3+……+an
Sn=1(1/2)^1+2(1/2)^2+3(1/2)^3+……+n(1/2)^n ①
1/2Sn= 1(1/2)^2+2(1/2)^3+……+(n-1)(1/2)^n+n(1/2)^(n+1) ②
①-②
1/2Sn=1/2+(1/2)^2+(1/2)^3+……+n(1/2)^n -n(1/2)^(n+1)
1/2Sn=1/2[1-(1/2)^n]/(1-1/2)-n(1/2)^(n+1)
Sn=2-(1/2)^(n-1)-n(1/2)^n