suku kelima dan suku ketujuh dari suatu barisan geometri berturut turut adalah 80 dan 320.suku kesembilan barisan tersebut adalh

U5 = ar^4 = 80
U7 = ar^6 = 320

ar^6 = 320
----------------
ar^4 = 80
r^2 = 4
r = 2

ar^4 = 80
a.2^4 = 80
a.16 = 80
a = 80/16
a = 5

Dit : U9?
=ar^8
=5.2^8
=5.256
=1280

diket : u5 = 80
u7 = 320
ditanya : u9...?
jawab : u5 = ar^4
=> 80 = ar^4 ... 1(persamaan satu)
u7 = ar^6
=> 320 = ar^6 ... 2
r = ar^6/ar^4
320/80 = r^
4 = r^
r = 2
a = ?
u5 = ar^4
80 = a.2^4
80 = a.16
a = 80/16
a = 5
jadi, u9 = ar^8 
u9 = 5.2^8
u9 = 5.256
u9 = 1280