Suku pertama dan kelima suatu barisan geometri berturut-turut 5 dan 80. Suku ke-9 barisan tersebut adalah...

U1 = a = 5
r = ⁴√80/5 = ⁴√16 = 2

U9 = a.r^8 = 5.2^8 = 5.256 = 1.280

a = 5
u5 = 80

r^4 = 80/5
r^4 = 16
r = 2


U9 = a.r^8
U9 = 5.2^8
U9 = 5.256
U9 = 1280

atau

U9 = U5.r^4
U9 = 80.2^4
U9 = 80.16
U9 = 1280