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2014-01-28T10:34:45+07:00
Deret geometri dg r = 2
log U₁ + log U₂ + log U₃ + log U₄ + log U₅ = 15 log 2
berdasarkan sifat logaritma, maka didapat :
log (U₁ . U₂ . U₃ .U₄ . U₅) = log 2¹⁵
U₁ x U₂ x U₃ x U₄ x U₅ = 2¹⁵
a x ar x ar² x ar³ x ar⁴ = 2¹⁵
a⁵ x r¹⁰ = 2¹⁵ 
a x r ² = 2³
a x 2² = 2³
a = 2
S₉ = a(r⁹ - 1) : (r - 1)
     = 2(2⁹ - 1) : (2 - 1)
     = 2(512 - 1) : 1
     = 2(511)
     = 1022


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