Sn=n^2+2n+5
Sn-1=(n-1)^2+2(n-1)+5=n^2-2n+1+2n-2+5
=n^2+4
因为：
Sn=a1+a2+a3+…+an-1+an
Sn-1=a1+a2+a3+…+an-1
所以错位相减：an=Sn-Sn-1
则：an=Sn-Sn-1=n^2+2n+5-(n^2+4)
=n^2+2n+5-n^2-4
=2n+1