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Bab Barisan dan Deret
Matematika SMP Kelas IX

Geometri
a] S2 = a x (r² - 1) / (r - 1) = 4
a x (r + 1) (r - 1) / (r - 1) = 4
a x (r + 1) = 4
a = 4/(r + 1)

S4 = a x (r⁴ - 1) / (r -1) = 40
a x (r² + 1) (r + 1) (r - 1) / (r - 1) = 40
a = 40 / ((r² + 1) . (r + 1))

a = a
40 / ((r² + 1) (r + 1)) = 4/(r + 1)
** coret (r + 1) **
** 40 : 4 = 10 **
r² + 1 = 10
r² + 1 - 10 = 0
r² - 9 = 0
(r + 3) (r - 3) = 0
r = -3 atau r = 3

a = 4/(r + 1)
a = 4/(-3 + 1)
a = -2

a = 4/(3 + 1)
a = 1

jadi, jika a = 1 maka r = 3 atau jika a = -2 maka r = -3

b] Sn = a x (rⁿ - 1) / (r - 1)
S6 = 1 x (3⁶ - 1) / (3 - 1)
S6 = 728/2
S6 = 364

S6 = -2 x (1 - (-3)⁶) / (1 -(-3) )
S6 = -2 x (1 - 729) / 4
S6 = - (-728) / 2
S6 = 364