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Root Test and Integral Test Question

  1. Nov 15, 2014 #1


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    Gold Member

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

    From K=4 to infinity the Σ (-1)^k (k/e^k)

    Converge or diverge?
    a) Ratio Test
    b) Root Test
    c) Integral Test
    d) Alternating series test

    2. Relevant equations

    3. The attempt at a solution

    For the alternating series test and ratio test I have the correct answer that it converges. These were fairly simple for me to proceed with.
    However, I am stuck on the Root test and Integral test.
    For the root test I DID get an answer, but it seems corrupt:

    Lim as n approaches infinity of (|(-1)^k (k/e^k|))^(1/k)
    With some simplification I narrowed it down to
    The lim as n-->infinity of (|n^(1/n)|/e)

    Which doesn't seem solvable ?

    And for the Integral test, I am seeing the answer requires imaginary numbers, etc. which we do not use in this class. Is it possible that the instructor did not realize this? Does this problem demand the use of imaginary numbers, etc? If so, I imagine I would be able to pass on this part.

    Thanks for any guidance.
  2. jcsd
  3. Nov 15, 2014 #2


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    2017 Award

    Staff: Mentor

    Root test: it is solvable. The numerator is a well-known limit problem with a standard answer, but here you do not need the exact limit - it is sufficient to find some upper bound, and that is easier to find.

    You can show absolute convergence instead of the (weaker) convergence. There, you don't get issues with complex numbers.
  4. Nov 15, 2014 #3


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    Ah, music to my ears. I see exactly what to do with the absolute convergence of the integral test. I took out the (-1)^k and then integrated to get a result o 5/e^4 which concludes absolute convergence.

    Now, I will try to work on the root test.

    Thank you for your guidance.
  5. Nov 17, 2014 #4


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    For the Root Test:

    I took the limit of the numerator and denominator. For the common limit of n^(1/n) the limit is 1. For e, the limit is the constant --> e.
    Thus, answer is 1/e and the limit is less then 1 meaning absolute convergence.

    Thank you for your guidance.
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