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anonymous

  • one year ago

Find S20 for the series -1 + -3 + -5 + -7 +... help?

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  1. Loser66
    • one year ago
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    You have formula for this, what is it? Notice: S means sum

  2. anonymous
    • one year ago
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    s=-1-2n

  3. Loser66
    • one year ago
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    I don't get. Use draw button to jot it down, please

  4. anonymous
    • one year ago
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    |dw:1439655417621:dw|

  5. Loser66
    • one year ago
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    https://www.mathsisfun.com/algebra/sequences-sums-arithmetic.html

  6. Loser66
    • one year ago
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    you need find d. What is d? the difference between 2 consecutive numbers.

  7. Loser66
    • one year ago
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    |dw:1439655734476:dw|

  8. anonymous
    • one year ago
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    -2

  9. anonymous
    • one year ago
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    s20=-39

  10. Loser66
    • one year ago
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    Your series starts from \(a_1 \) and ends at \(a_{20}\), right? that means you have 20 elements there, right? That is n =20 Now plug all in \(S_{20}= \dfrac{n}{2}(2a_1+(n-1)*d)\) What do you get?

  11. anonymous
    • one year ago
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    whats a1

  12. anonymous
    • one year ago
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    i got -420?

  13. Loser66
    • one year ago
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    a_1 is the first number: -1

  14. anonymous
    • one year ago
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    -400

  15. Loser66
    • one year ago
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    yes, hihihi... I am sorry.

  16. anonymous
    • one year ago
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    -400?

  17. Loser66
    • one year ago
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    yes

  18. Loser66
    • one year ago
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    Let's test

  19. Loser66
    • one year ago
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    hit your calculator -1-3-5.......-39=?

  20. anonymous
    • one year ago
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    yep

  21. Loser66
    • one year ago
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    yES, I got -400

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spraguer (Moderator)
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is replying to Can someone tell me what button the professor is hitting...

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