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8 May, 22:13

What is the sum of the first five terms of a geometric series with a1 = 10 and r = 1/5?

Express your answer as an improper fraction in lowest terms without using spaces.

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Answers (2)
  1. 8 May, 22:15
    0
    A 1 = 10

    a 2 = 10 * 1/5 = 2

    a 3 = 2 * 1/5 = 2/5

    a 4 = 2/5 * 1/5 = 2/25

    a 5 = 2/25 * 1/5 = 2/125

    S 5 = a 1 + a 2 + a 3 + a 4 + a 5 = 10 + 2 + 2/5 + 2/25 + 2/125 =

    = 1250/125 + 250/125 + 50/125 + 10/125 + 2/125 = 1562 / 125
  2. 8 May, 22:28
    0
    Formula : Sn=a1 (1-r^n) / 1-r

    S5=10 (1-1.5^5) / 1-1.5

    10 (1-7.8) / - 0.5

    10 (-6.8) / - 0.5

    -68/-0.5 = 136
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