a1a2=6
a2a3=3a1a2=18,a3=18/a2=6
a3a4=3a2a3=54,a4=9
同样可得:a5=18,a6=27
A2n-1A2n-2=3*(A2n-3A2n-2),
A2n-3=A2n-1/3
同样可得:
A2n-2=A2n/3
A2n+1=3A2n-1
A2n+2=3A2n
Bn-1*Bn+1=(2A2(n-1)-1+A2(n-1))*(2A2(n+1)-1+A2(n+1))
=(2A2n-3+A2n-2)*(2A2n+1+A2n+2)
=(1/3)*(2A2n-1+A2n)*(3)*(2A2n-1+A2n)
=(2A2n-1+A2n)^2
=Bn^2
所以Bn成等比数列
按你修改的也是同样的证明,从证明过程中可以发现,与前面有没有2无关!
Bn-1*Bn+1=(A2(n-1)-1+A2(n-1))*(A2(n+1)-1+A2(n+1))
=(A2n-3+A2n-2)*(A2n+1+A2n+2)
=(1/3)*(A2n-1+A2n)*(3)*(A2n-1+A2n)
=(A2n-1+A2n)^2
=Bn^2