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Sequences and Series Help

  1. Apr 24, 2007 #1
    This was the extra credit question on a quiz I had today, I am very anxious to find out the answer.

    1. The problem statement, all variables and given/known data

    Find the apparent Nth term of the sequence
    2,-5,10,-17 ...... n

    2. Relevant equations

    Not sure really on this
    an = .....

    3. The attempt at a solution


    The sequence starts with 2 and then increases by sequential odd numbers and alternates positive and negative.

    Thanks very much
  2. jcsd
  3. Apr 24, 2007 #2


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    Homework Helper
    Gold Member

    As far as I can tell, your answer is correct. Nice Job!
  4. Apr 25, 2007 #3
    Actually I was thinking about it and my answer is only correct for the first two terms, after that it's in accurate, the real answer is

    1+n^2 , I'm hoping that my teacher is swamped with grading and will only check the first two terms.
  5. Apr 27, 2007 #4
    you forgot the (-1)^(n-1) part.
    And your original answer does not wort even for the first two terms, just for the first one.

    when u take n=2 you will get

    Last edited: Apr 27, 2007
  6. Apr 27, 2007 #5
    It looks to me like the nth term is the sum of the first n primes times (-1 raised to the power of n+1)
  7. Apr 28, 2007 #6
    Im positive my answer works for the first two

    (2+(4-1)) * (-1)^(2-1) = -5
  8. Apr 28, 2007 #7


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    But that's not what you wrote! In your first post you wrote
    (n)+(2n-1)*(-1)^(n-1) where the (-1)^(n-1) is multiplied only by the second term, not the first.

    In any case, "correct for 2 terms" is still not good enough. For the three terms given, I see"subtract 7, then add 5, then subtract seven, then add five, ..."
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