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infinitemoes

Please help! What is the sum of the series? 20 on top and n-1 on the bottom of ∑(5-n)

  • 10 months ago
  • 10 months ago

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  1. Dido525
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    did you mean n=1?

    • 10 months ago
  2. Dido525
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    @infinitemoes

    • 10 months ago
  3. infinitemoes
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    no it's a n-1 on the bottom of the sigma.

    • 10 months ago
  4. mrbarry
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    \[\sum_{n-1}^{20} (5-n)\]

    • 10 months ago
  5. infinitemoes
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    yes exactly like that

    • 10 months ago
  6. mrbarry
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    It should be n=1 I am certain. If we did this "manually" we would have 4 + 3 + 2 + ... Right?

    • 10 months ago
  7. infinitemoes
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    alright..

    • 10 months ago
  8. mrbarry
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    This is an arithmetic sequence b/c we are adding -1 over and over. There is a formula to solve this, Arithmetic Series Formula. Do you know it?

    • 10 months ago
  9. infinitemoes
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    I think I got the answer. -110

    • 10 months ago
  10. mrbarry
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    Let's see. The formula says: 1/2 the number of items times (first term + last term) 1/2 (20) (4 + -15) 10 (-11) Good job.

    • 10 months ago
  11. infinitemoes
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    thank you! :)

    • 10 months ago
  12. mrbarry
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    You are welcome. All done? Thanks for the medal.

    • 10 months ago
  13. infinitemoes
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    Well I do have a couple more questions, but I'm gonna post them separate to this post.

    • 10 months ago
  14. mrbarry
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    Of course.

    • 10 months ago
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