Sukuke-3 dan suku ke-5 suatu barisan geometri berturut turut adalah 20 dan 80 suku ke-9 barisan tersebut adalah

r = 2
a =  5
[tex]U_{9} [/tex] = 5 [tex]2^{9-1} [/tex]
[tex]U_{9} [/tex] = 5 × [tex]2^{8} [/tex]
U9 = 5 × 256
U9 = 1280

u3 = 20
u5 = 80
u9 = ?

un = ar^(n-1)
20 = ar^(3-1)
20 = ar^2 .......(1)

80 = ar^(5-1)
80 = ar^4 ........(2)


80 = ar^4/20 = ar^2
4 = r^2
r^2 = 4
r = 2

20 = ar^2
20 = a × 2^2
20 = a × 4
a × 4 = 20
a = 20/4
a = 5

u9 = ar^(n-1)
u9 = 5 × 2^(9-1)
u9 = 5 × 2^8
u9 = 5 × 256
u9 = 1280