Sum of n terms of G.P. is given by = {a(rn - 1)}/(r - 1)
ATQ
{a(r8 - 1)}/(r - 1) = 5{a(r4 - 1)}/(r - 1)
(r4)2 - 1 = 5(r4 - 1)
(r4 + 1)(r4 - 1) = 5(r4 - 1)
r4+1 = 5
r2 = 2 0r -2 As square can't be neagtive
r2 = 2
Common ratio r = √2 or -√2