a1=3，an=3+2(n-1)=2n+1Sn=1/2*(2n+1+3)*n=n(n+2)1/Sn=1/2*(1/n-1/(n+2))2Tn=1-1/3+1/2-1/4+1/3-1/5+...+1/(n-2)-1/n+1/(n-1)-1/(n+1)+1/n-1/(n+2)=1+1/2-1/(n+1)-1/(n+2)Tn=n(3n+5)/[4(n+1)(n+2)]
先求出an，再求出sn一步步做其实很简单
