a1=1/4,b1=3/4
bn+1=bn/(1-an)(1+an)=1/(1+an)
bn= 1/[1+a(n-1)]
把上面的bn带入1/（bn -1）= 1/{1/[1+a(n-1)] -1} = -1- 1/[a(n-1)]
[a(n-1)] =1-b(n-1)
所以 1/（bn -1）= -1+ 1/[b(n-1) -1]
(1/bn-1)-1/[b(n-1) -1] =-1
1/(bn-1)是等差数列,1/(b1-1)=-4
1/(bn-1)=-4+(-1)*(n-1)=-n-3
1/(bn-1)=-1/an=-n-3
an=1/(n+3)