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U2 = 30 (dibagi)
r³ = 8
r = ∛8
r = 2
Nilai a :
U2 = a.r
30 = a.2
a = 30/2 = 15
Sn = a(r^n-1) / r-1
S7 = 15(2^7-1) / 2-1
S7 = 15(128-1)
S7 = 15(127)
S7 = 1905
Jadi, jumlah 7 suku pertama adalah 1905
U5 = a.r^4 = 240
a.r.r³ = 240
30.r³ = 240
r³ = 240/30
r³ = 8
r = ∛8 = 2 ----> a.r = 30
a.2 = 30
a = 30/2 = 15
Sn = a{(r^n) - 1} / (r - 1)
S7 = 15 {2^7) - 1} / (2 - 1)
= 15.{128 - 1} / 1
= 15 (127)
= 1905