Diketahui deret geometri dengan suku ke 3 dan ke 5 berturut turut adalah 2/3 dan 2/27.jika rasio > 0,jumlah sampai 4 suku deret tsb adalah..
a.52/9
b.56/9
c.74/9
d.79/9
e.80/9
a.52/9
b.56/9
c.74/9
d.79/9
e.80/9
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ar² = 2/3
U5:
ar⁴ = 2/27
ar².r² = 2/27
2/3r² = 2/27
r² = 2/27 ÷ 2/3
r² = 2/27 × 3/2
r² = 1/9
r = √1/9
r = 1/3
ar² = 2/3
a(1/9) = 2/3
a = 2/3 ÷ 1/9
a = 2/3 × 9/1
a = 18/3
S4 = a(r⁴ - 1) / r - 1
S4 = 18/3 ((1/3)⁴ - 1) / (1/3) - 1
S4 = 18/3 ( 1/81 - 1 ) / 2/3
S4 = 9( 80/81)
S4 = 80/9
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