Three positive integers are such that the middle one exceeds the first by 4 and their product is 1536.
a. Taking the three integers to be a,a+4 and a+8, form an equation in a and solve it to obtain the integers.
b. Show that the three integers form an arithmetic progression and state its common difference.
c. Using the progression obtained in part (ii), write the first five terms.
d. Find the sum of the first 20 terms of this arithmetic progression.