Sn = a(rn-1)/r-1 , jika r>1
= a(1-rn)/1-r , jika r<1
S∞ = a/(1-r)
Ket:
a = suku awal
r = rasio
n = banyak suku
contoh:
1. Diketahui
barisan geometri dengan U3 = 27 dan U5 = 243. Berapakah 6 suku pertama deret
tersebut?
Solusi :
Solusi :
1. U3 =
a . r3-1 = a . r2 = 27 27
= U1 . (3)3-1
U5 =
a . r5-1 = a . r4 = 243 27
= U1 . 32
27 =
U1 . 9
U5/U3
= a . r4 / a . r2 = 243/27 U1
= 27 : 9 = 3
r2 =
9 U1
= 27 : 9 = 3
r = 3
S15 = 3 ( 36 - 1) / 3-1 = 3 (729-1) / 2 = 3
(728) /2 = 1092
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