先令n=1，求出a1=s1则n>=2时an=Sn-Sn-1再合并
Sn=n^2+2nS(n-1)=(n-1)^2+2(n-1)=n^2-2n+1+2n-2=n^2-1an=Sn-S(n-1)=n^2+2n-(n^2-1)=2n+1
