diketahui barisan geometri bahwa U3=4 dan U9=256 maka U12 adalah

u9/u3 = ar^8/ar² = r^6
256/4 = r^6
64 = r^6
r = 2

u12 = ar^11 = ar^(8+3) = ar^8 x r³ = 256 x 2³
= 256 x 8 = ..... itung sndiri

Un = U1 + (n - 1)b

U3 = U1 + (3 - 1)b
U3 = U1 + 2b

U9 = U1 + (9 -1)b
U9 = U1 + 8b

Eliminasi

U1 + 8b = 256
U1 + 2b = 4
___________ _
6b = 252
b = 252/6
b = 42

U1 + 2(42) = 4
U1 + 84 = 4
U1 = 4 - 84
U1 = 80

U12 = 80 + (12 - 1)42
U12 = 542