An=Sn- Sn-1 = n²+n-(n-1)²-n+1=2nBn = ( 1/n - 1/(n+1) )/4 Tn= 1/4 - 1/(4n+4)
n=1，s1=2=a1n≥2，an=sn-s（n-1）=2n
