已知数列{an}中,an=2n+2^n-1,则前n项和Sn=
问题描述：
已知数列{an}中,an=2n+2^n-1,则前n项和Sn=
答
Sn=a1+a2+``````+an
=2x1+2^1-1+````+2n+2^n-1
=2x(1+2+`````+n)+(2^1+```+2^n)-n
=n^2+n+2^(n+1)-2-n
=2^(n+1)+n^2-2