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anonymous

  • one year ago

What is the sum of the arithmetic sequence 3, 9, 15..., if there are 36 terms?

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  1. welshfella
    • one year ago
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    Sum of n terms = (n/2)[2a + d(n-1)] where d = common difference and a = first term

  2. welshfella
    • one year ago
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    common difference d = 9- 3 = 6

  3. anonymous
    • one year ago
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    right then what do I do?

  4. welshfella
    • one year ago
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    well plug in the values n = 36, d = 6 and a = 3

  5. welshfella
    • one year ago
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    plug these in and work it out

  6. welshfella
    • one year ago
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    = (36/2)[2*3 - 6(36-1)]

  7. welshfella
    • one year ago
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    sorry that should be a + before the 6 not -

  8. anonymous
    • one year ago
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    you're all good so then I have to solve that?

  9. welshfella
    • one year ago
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    do the parts in the parentheses first

  10. anonymous
    • one year ago
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    (18)[35] is what I got but that doesn't seem like it makes sense at all

  11. welshfella
    • one year ago
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    36/2 = 18 36 - 1 = 35

  12. welshfella
    • one year ago
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    no thats not right its 18(6 + 6*35) do the stuff in the parentheses first then finall y multiply by 18

  13. anonymous
    • one year ago
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    7560?

  14. welshfella
    • one year ago
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    multiply 6 by 35 before adding the 6

  15. anonymous
    • one year ago
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    actually 3888

  16. welshfella
    • one year ago
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    no i think you are doing things in the wrong order remember PEDMAS Parentheses Exponent Division Multiplication Add Subtract thats the order of operations

  17. welshfella
    • one year ago
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    yes 3888 is correct 18(6 + 6*35 = 18*(6 + 210) = 18 * 216 = 3888

  18. anonymous
    • one year ago
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    thank you!

  19. welshfella
    • one year ago
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    PEDMAS is very important

  20. welshfella
    • one year ago
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    yw

  21. welshfella
    • one year ago
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    note i worked out 6 + 210 before i multipilied by 18 because 6 + 210 was in the parentheses

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