Sn=n(2n-1)an,S(n+1)=(n+1)[2(n+1)-1]a(n+1)=(n+1)[2n+1]a(n+1)S(n+1)-Sn=a(n+1)即(n+1)[2n+1]a(n+1)-n(2n-1)an=a(n+1)n(2n-1)an=（2n^2+3n）a(n+1)(2n-1)an=（2n+3）a(n+1)a(n+1)=(2n-1)/（2n+3）an a2=1/5*1/3=1/1...