a4/a1=q³=8
q=2
∴an=2×2^(n-1)=2^n
∴bn=1/{ log2(2^n) × log2[2^(n+1)] } = 1/ [n(n+1)]
∵bn=1/ [n(n+1)]= 1/n - 1/(n+1)
∴Sn= (1 - 1/2) + (1/2 - 1/3)+ ( 1/3 - 1/4) +…… + [1/n - 1/(n+1) ]
= 1 - 1/(n+1)
=n/(n+1)＜1