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anonymous

  • one year ago

Find S8 for the series -2 + -10 + -50 + -250 +... i got 781248

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  1. anonymous
    • one year ago
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    @Michele_Laino

  2. Michele_Laino
    • one year ago
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    it is a geometric sequence whose constant is q=5 and first term is a1=-2

  3. anonymous
    • one year ago
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    okay. and i plug it to the formula and got 781248

  4. Michele_Laino
    • one year ago
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    we have to apply this formula: \[\Large {S_8} = {a_1}\frac{{1 - {q^8}}}{{1 - q}} = 1 \cdot \frac{{1 - {5^8}}}{{1 - 5}} = ...?\]

  5. anonymous
    • one year ago
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    no wait it shud be 195312

  6. anonymous
    • one year ago
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    a1 should be -2 not 1

  7. Michele_Laino
    • one year ago
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    oops.. \[\Large {S_8} = {a_1}\frac{{1 - {q^8}}}{{1 - q}} = \left( { - 2} \right) \cdot \frac{{1 - {5^8}}}{{1 - 5}} = ...?\]

  8. anonymous
    • one year ago
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    so did i get the right anwser?

  9. Michele_Laino
    • one year ago
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    it is -195312 since you are summing all negative terms

  10. anonymous
    • one year ago
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    okay thank u

  11. Michele_Laino
    • one year ago
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    :)

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