suatu deret geometri di ketahui V2=8 dan V5=1 tentukan jumlah suku pertamanya

dik: U2 = 8; U5 = 1;
dit: S5?
jawab:
U2 = ar = 8
a = 8/r

U5 = ar⁴ = 1
(8/r) r⁴ = 1
8r³ = 1
r³ = 1/8
r = 1/2

a = 8/r
a = 8/(1/2)
a = 16

S5 = a(1 - r^n) / (1 - r)
S5 = 16(1 - (1/2)⁴) / (1 - (1/2))
S5 = 16(1 - (1/16)) / (1/2)
S5 = 16(15/16) . 2
S5 = 15 . 2
S5 = 30