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8 September, 13:24

What is the sum of the first nine terms of the geometric series where a1 = 7 and r = - 3?

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  1. 8 September, 13:36
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    Sum of terms of a geometric series

    Sn = a1 (r^n-1) / (r-1)

    a1 = 1st term = 7

    n = number of terms = 9

    r = common ratio = - 3

    Sn = 7 ((-3) ^8-1) / (-3-1) = - 11480

    Hence the sum is - 11480.
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