The recursive formula is always expressed in terms of a previous term. In the given scenario,
first term, f(1) = 300
This amount reduces by 1/2 each month. Thus, common ratio is 1/2
f(n - 1) refers to the term before n at every given time. Recall, n = number of months. Thus, the recursive formula is
f(n) = 1/2f(n - 1)
Where f(n - 1) is the previous term
The formula for the explicit formula is
an = a1 * r^(n - 1)
Thus,
an = 300 * 1/2^(n - 1)