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anonymous

  • one year ago

can someone please help me... I need to pass math. what is the sum of the series? A. -220 B. -110 C. -95 D. 110

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  1. anonymous
    • one year ago
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    \[\sum_{n-1}^{20} (5-1)\]

  2. anonymous
    • one year ago
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    that 1 in the parenthesis is supposed to be an n

  3. anonymous
    • one year ago
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    I did the equation but when I add all the number up I get a weird number not one that i need.

  4. anonymous
    • one year ago
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    That's an arithmetic sequence because it's linear. You can use this formula |dw:1435279685117:dw|

  5. anonymous
    • one year ago
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    \[\sum_{n=1}^{20} (5-n)\]?

  6. anonymous
    • one year ago
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    I tried this but when I add the numbers I don't get the answer that i need... Could you help further on please.

  7. anonymous
    • one year ago
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    you can do @peachpi way or you can break it in two \[\sum_{n=1}^{50}5-\sum_{n=1}^{20}n\]

  8. anonymous
    • one year ago
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    oops \[\sum_{n=1}^{20}5-\sum_{n=1}^{20}n\]

  9. anonymous
    • one year ago
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    I did 5 -1 -1 5 - 2 - 1 5 - 3 - 1 and so on

  10. anonymous
    • one year ago
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    the first summation is \(20\times 5\) and for the second one it is \[\frac{20\times 21}{2}\]

  11. anonymous
    • one year ago
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    use @peachpi formula, you do not want to add up twenty numbers

  12. anonymous
    • one year ago
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    okay I will try

  13. anonymous
    • one year ago
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    @peachpi can you write you formula out I dont really understand it

  14. anonymous
    • one year ago
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    your formula is 5 -n. The sum goes from 1 to 20, so plug them in. What do you get when you sub in 1 for n? What do you get when you sub in 20 for n?

  15. anonymous
    • one year ago
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    5 - 1 = 4 5 - 20 = -15

  16. anonymous
    • one year ago
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    right...

  17. anonymous
    • one year ago
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    |dw:1435280659076:dw|

  18. anonymous
    • one year ago
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    -11

  19. anonymous
    • one year ago
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    5*-11

  20. anonymous
    • one year ago
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    -55

  21. anonymous
    • one year ago
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    \[\frac{20}{2}\neq 5\]

  22. anonymous
    • one year ago
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    10

  23. anonymous
    • one year ago
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    @satellite73

  24. anonymous
    • one year ago
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    so then 10 * -11

  25. anonymous
    • one year ago
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    -110

  26. anonymous
    • one year ago
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    yes

  27. anonymous
    • one year ago
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    thank you so much.

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