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26 June, 02:00

Find the sum of the geometric sequence - 3, 15, - 75, 375, ... when there are 9 terms and select the correct answer below.

Choices:

A. - 976,563

B. 976,563

C. 1,464,843

D. 976,562

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Answers (1)
  1. 26 June, 03:03
    0
    Sn=sum of the n terms of the geometric sequence.

    a = the first term

    r=the common ratio

    n=numbers of terms.

    Sn=a[ (1-r^n) / (1-r) ]

    In this case:

    a=-3

    r=a₂/a₁=15/-3=-5

    n=9

    S₉=-3[ (1 - (-5) ⁹) / (1 - (-5)) =

    S₉=-3 (1+1953125) / 6) =

    S₉=-3 (1953126/6) =

    S₉=-3 (325521)

    S₉=-976563

    Answer: A. - 976563
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