5. Pada barisan geometri U1 = 81 dan U5 = 1 . Berapakah jumlah 8 suku pertama

Mapel: Matematika
Bab: Barisan dan Deret

Diketahui:
Barisan Geometri
U1 = 81
U5 = 1

Ditanyakan:
S8?

Jawab:
U1 = 81
a = 81

U5 = 1
ar^4 = 1
81.r^4 = 1
r^4 = 1/81
r^4 = (1/3)^4
r = 1/3

Sn = (a (1 - r^n)) / (1 - r)
S8 = (81 (1 - (1/3)^8)) / (1 - 1/3)
= (81 (1 - 1/6561)) / (2/3)
= (81 (6560/6561) (3/2)
= 121,48