数列an的前n项和sn=n²+3n,求an的通项公式
问题描述：
数列an的前n项和sn=n²+3n,求an的通项公式
答
Sn=n^2+3n
S(n-1)=(n-1)^2+3(n-1)
=n^2-2n+1+3n-3
=n^2+3n+(-2n-2)
Sn-S(n-1)
=an
=n^2+3n-n^2-3n+2n+2
=2n+2