In an arithmetic sequence the difference between consecutive terms is a constant
In a geometrical serial , the division between consecutive terms is constant
In this problem looking at formula
d(n)/(d(n-1) = n , so this means is a geometrical serial
and the first 5 terms are
d(1) = 1. d(2) = 2•1 = 2. d(3)= 3•d(2)= 3•2 = 6. d(4)= 4•d(3)= 4•6= 24
d(5)= 5•d(4)= 5•24 = 120
