an=Sn-Sn-1=n(2n-1)an-(n-1)(2n-3)an-1(2n+1)(n-1)an=(n-1)(2n-3)an-1x(2n+1)an=(2n-3)an-1即：5xa2=a17xa3=3a29xa4=5a3.(2n+1)an=(2n-3)an-1所有等式左右边分别相乘,约去a2,an-1,及5,7,（2n-3),(2n-1)(2n+1)an=3a1=...