a(1)=1,
a(n+1) = (2n-1)a(n)/(2n+3),
(2n+3)a(n+1) = (2n-1)a(n),
(2n+3)(2n+1)a(n+1) = (2n+1)(2n-1)a(n).
[2(n+1)+1][2(n+1)-1]a(n+1) =(2n+1)(2n-1)a(n),
{(2n+1)(2n-1)a(n)}是首项为3*1a(1)=3,的常数数列.
(2n+1)(2n-1)a(n) = 3,
a(n)= 3/[(2n+1)(2n-1)]
构造常数列法的核心是,使得等号两边的递推关系分别表达了k=n+1,和k=n时,某函数b(k)的形式