Suatu barisan geometri 81,27,9,3 maka jumlah n suku pertamanya adalah.....

Kelas 8 Matematika
Bab Barisan dan Deret Bilangan

r = U2/U1
r = 27/81
r = 1/3

Sn = a . (1 - rⁿ) / (1 - r)
Sn = 81 . (1 - (1/3)ⁿ) / (3/3 - 1/3)
Sn = 81 . (1 - (1/3)ⁿ) / (2/3)
Sn = 81 . (1 - (1/3)ⁿ) . 3/2
Sn = (243 . (1 - (1/3)ⁿ))/2