The sequence `(:a_(n-1):),n in N` is an arithmetical progression and `d` is its common difference.If `lim_(n rarr oo)(1-(d^(2))/(a_(1)^(2)))(1-(d^(2))/(a_(2)^(2)))cdots cdots(1-(d^(2))/(a_(n)^(2)))` converges to `(1)/(4)` and `a_(1)=8` then find the value of `d`