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S6 = 42
S6 < S3 ⇒ deret geometri turun ( r < 1 )
Un = arⁿ⁻¹
Sn = a (1 - rⁿ)
Sn = a(1 – rⁿ) / (1 – r)
S6/S3 = [a(1 – r⁶)/(1 – r)] / [a(1 – r³)/(1 – r)]
--------------------------------------... kedua ruas dibagi a/(1 – r)
S6 / S3 = (1 – r⁶) / (1 – r³)
Ingat rumus: (a² – b²) = (a + b) (a – b)
(1 – r⁶) = (1 + r³)(1 – r³)
42 / 48 = (1 + r³)(1 – r³) / (1 – r³)
7 / 8 = (1 + r³)
dikalikan silang
8(1 + r³) = 7
8 + 8r³ = 7
8r³ = -1
r³ = -⅛
r = akar pangkat tiga (-⅛)
r = -½
S3 = a(1 – r³)/(1 – r)
48 = a[1 – (-½)³] / [1 – (-½)]
48 = a[1 – (-⅛)] / (1 + ½)
48 = a(1 + ⅛) / (1 + ½)
48 = a(9/8) / (3/2)
48 = a(3/4)
a = 4/3 * 48
a = 64
Sn = Sn-1 + Un
S7 = S6 + U7
S7 = 42 + ar⁶
S7 = 42 + 64*(-½)⁶
S7 = 42 + 64*(1/64)
S7 = 42 + 1
S7 = 43