s1=a1=3+1+1=5
当n>=2时
an=Sn-Sn-1
=3n^2+n+1-(3(n-1)^2+n-1+1)
=3n^2-3(n-1)^2+1
=6n-2
代入n=1显然不满足
所以an =6n-2 n>=2
n=1 a1=5
要分开写