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Homework Help: Cosine question

  1. Jan 31, 2007 #1
    1. The problem statement, all variables and given/known data
    While solving the integral of a wave function,I came across the term cos(n*pi) , where n is an integer. Is that term equal to +1 or -1 (I know that it could be either one depending on whether n is odd or even) but how do I proceed with the integral?

    2. Relevant equations


    3. The attempt at a solution

    cos (n*pi) is 1 if n is even and -1 if n is odd...but how do proceed?

    thanks a lot
  2. jcsd
  3. Jan 31, 2007 #2

    Tom Mattson

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    Why does n have to be +1 or -1? Why can't it be 2, for instance?
  4. Jan 31, 2007 #3
    n could be any integer, but the whole term cos (n*pi) will have to be +1 or -1
  5. Jan 31, 2007 #4

    Tom Mattson

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    OK, so you're trying to integrate and you have somewhere a factor of [itex]\cos(n\pi)[/itex], where [itex]n[/itex] is an integer. Is that right? In that case you can just replace [itex]\cos(n\pi)[/itex] with [itex]\pm 1[/itex].

    This looks pretty straightforward to me. It might help me understand why this is giving you trouble if you typed out the integral that you are trying to do, what you have done, and where you got stuck.
  6. Jan 31, 2007 #5
    You can write the term as

    \cos(n\pi) = (-1)^{n}

    It doesn't really help that much, but you see this a lot. Since this is a constant term you can just proceed to perform the integral you want to evaluate.
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