The n^th term of a GP is a_n = ar^n-1

It’s given in the question that 5^th term of the GP is 80 and 8^th term of GP is 640.

So, a₅ = ar⁴ = 80 → (1)

a₈ = ar⁷ = 640 → (2)

→

Common ratio, r = 2,

ar⁴ = 80

16a = 80

a = 5

The required GP is of the form a, ar, ar², ar³, ar⁴….

First term of GP, a = 5

Second term of GP, ar = 5 x 2 =10

Third term of GP, ar² = 5 x 2² = 20

Fourth term of GP, ar³ = 5 x 2³ = 40

Fifth term of GP, ar⁴ = 5 x 2⁴ = 80

And so on...

The required GP is 5, 10, 20, 40, 80…