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anonymous

  • one year ago

Find the sum of the geometric sequence –3, 15, –75, 375, … when there are 8 terms and select the correct answer below.

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  1. anonymous
    • one year ago
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    what is r?

  2. anonymous
    • one year ago
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    What is what?

  3. anonymous
    • one year ago
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    the common ratio

  4. anonymous
    • one year ago
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    Is it 5?

  5. anonymous
    • one year ago
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    Or -5?

  6. anonymous
    • one year ago
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    -5 looks right

  7. SolomonZelman
    • one year ago
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    \(\large\color{black}{ \displaystyle {\rm S}_{\rm n}=\frac{a_1\times \left(1-{\rm r}^{\rm n}\right)}{1-{\rm r}} }\) this is the formula, for the sum of a geometric sequence that starts from some \(a_1\) and has n terms.

  8. SolomonZelman
    • one year ago
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    you need to find r, identify the \(a_1\) that you are given, and tell me the number of terms (which you plug in for n). plug everything into the formula, and calculate.

  9. anonymous
    • one year ago
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    I think I did something wrong in the equation because I got 24,414,063

  10. SolomonZelman
    • one year ago
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    holly molly.....

  11. SolomonZelman
    • one year ago
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    yeah it is wrong

  12. SolomonZelman
    • one year ago
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    tell me what was your common ratio (r)? what was your number of terms? what is the first term?

  13. anonymous
    • one year ago
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    My common ratio was -5 The number of terms was 8 The first term was -3

  14. SolomonZelman
    • one year ago
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    yes

  15. anonymous
    • one year ago
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    \((1-(r^n))\) is what I meant

  16. SolomonZelman
    • one year ago
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    \(\large\color{black}{ \displaystyle {\rm S}_8=\frac{ (-3)\times \left(1-(-5)^8\right)}{1-(-5)} }\) Click \(\bf\color{blue }{\href{http:///www.wolframalpha.com/input/?i=%28-3+%C3%97+%281+-+%28-5%29%5E8%29%29%2F%281-%28-5%29%29}{here}}\) to check the result.

  17. anonymous
    • one year ago
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    My common ratio was -5 The number of terms was 8 The first term was -3

  18. anonymous
    • one year ago
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    Is the answer 195,312??

  19. SolomonZelman
    • one year ago
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    yes, that is the answer.

  20. anonymous
    • one year ago
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    Thank you so much!

  21. SolomonZelman
    • one year ago
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    anytime !

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