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anonymous

  • one year ago

what is the sum of the geometric sequence -3 , 18, -108 if there are 7 terms?

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  1. anonymous
    • one year ago
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    The answer choices are: -719,835 -119,973 119,973 719,835

  2. anonymous
    • one year ago
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    Yes

  3. anonymous
    • one year ago
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    Noo

  4. anonymous
    • one year ago
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    There are four answer choices

  5. welshfella
    • one year ago
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    Sum of n terms = a1 r^n - 1 ------- r - 1 whre a1 = first term and r = common ratio

  6. welshfella
    • one year ago
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    so plug in a1 = 8 , r = -6 and n = 7

  7. welshfella
    • one year ago
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    sorry a1 = -3

  8. anonymous
    • one year ago
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    So it's -3(-6)^7-1 over -6-1?

  9. welshfella
    • one year ago
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    not exactly - work out the (-6)^7 first then subtract 1 then divide by -7 finally multiply by -3

  10. anonymous
    • one year ago
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    I got -119,973 is that right?

  11. welshfella
    • one year ago
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    I should have written the (r^n - 1) like that - in parentheses

  12. welshfella
    • one year ago
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    yes

  13. anonymous
    • one year ago
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    Okay thank you very much

  14. welshfella
    • one year ago
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    yw

  15. welshfella
    • one year ago
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    oh = press best response

  16. anonymous
    • one year ago
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    I did

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