an=1/2^n+3n-2
前n项和Sn
=(1/2+1/2^2+1/2^3+……+1/2^n)+3(1+2+3+……+n)-2n
=1/2*(1-1/2^n)/(1-1/2)+3*n(n+1)/2-2n
Sn=1-1/2^n+n(3n-1)/2
解毕