Let the common ratio of geometric series an be q, then there is

A5-a 1= 15, then a1(Q4-1) =15 (1).

A4-a2=6, then A2 (Q 2- 1) = 6 (2).

(1)/(2) de: (Q2+1)/q = 5/2q = 2 or 1/2, a 1= 1 or-16.

A3 = a1q 2 =1× 2 2 = 4 or a3 = a1q 2 =-16× (1/2) 2 =-4.

Let the first term of geometric series be a 1 and the common ratio be q, then there is

A5 = A 1Q 4 = 8, A7 = A 1Q 6 = 2 to get q= 1/2 or-1/2, a 1= 128.

Then an = a1q (n-1) =128× (1/2) (n-1) = (1/2) (n-8).