Question

Deret geometri S2=6, S4=30, S5??

Answer 1

S2 = a(r^2 -1)/(r-1) = 6
S4 = a(r^4 -1)/(r-1) = 30
..
S4/S2 = (r^4-1)/(r^2-1) = 5
(r^2-1)(r^2 +1)/(r^2-1) = 5
r^2 +1 = 5
r^2 = 4
r = 2 atau r = -2
krn jumlahnya makin naik ambil r >0 --> r = 2
..
S2 = a(r^2-1)/(r-1) = 6
a(4-1)/(2-1) =6 ---> 3a = 6 --> a= 2
.
S5 =  a(r^5 -1)/(r-1) = 2(2^5 -1)/(2-1) = 2(31)/(1) = 62 
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