(1) bn+1=(1-an)/(1-an^2)=1/a+an,1/(bn+1 +1)=-(an+1)/an,1/(bn -1)=-1/an ,-=-1
(2)b1=3/4
1/(bn+1 -1)-1/(bn -1)=-1
1/(bn -1)-1/(bn-1 -1)=-1
...累加
(1) bn+1=(1-an)/(1-an^2)=1/a+an,1/(bn+1 +1)=-(an+1)/an,1/(bn -1)=-1/an ,-=-1
(2)b1=3/4
1/(bn+1 -1)-1/(bn -1)=-1
1/(bn -1)-1/(bn-1 -1)=-1
...累加