Missing number: Between 72 and 18, there should be 36. Therefore, the series should look like this:
72, 36, 18, 9, 9/2 9/8, ...
This is a geometric sequence in which;
a = 72
r = 1/2
Therefore, explicit formula is;
a(n) = a(1/2)^n
Since,
r lies between 1 and -1, the sequence converges at a/(1-r). That is,
=> a/(1-r) = 72/(1-1/2) = 72/(1/2) = 72*2 = 144
