1. The 5th, 9thand16th terms of a linear sequence are consecutive terms of an exponential sequence

Question

GP. Find the common difference in terms of the first term�

Answer 1

(a+8d)/(a+4d) = (a+15d)/(a+8d)
(a+8d)(a+8d) = (a+4d)(a+15d)
a^2+16ad+64d^2 = a^2+19ad+60d^2
4d^2 = 3ad
4d = 3a
d = 3a/4

Answer 2

I do not still understand