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Proving this summation?

  1. Jan 12, 2017 #1
    1. The problem statement, all variables and given/known data
    Picture has been uploaded with question.

    2. Relevant equations

    3. The attempt at a solution

    I have literally never seen anything like this in my life. I'm in mathematical physics. I looked up de Moivre's formula and I guess this comes from a course in complex variables? I don't know why this is a homework question given that nobody in my class has actually taken it, but whatever. I need to be able to prove this. Can you please help me out, where should I start? I am familiar with geometric series and I guess I can reference De Moivre's formula. That's where I'm at. My initial thought was that this had something to do with a taylor expansion, but I know cos(x)'s taylor expansion and I don't think that has anything to do with this.

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  2. jcsd
  3. Jan 12, 2017 #2


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    Write out de Moivre's formula and see if it has the same form as one of the summands you're given.
  4. Jan 12, 2017 #3


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    Start by posting an image that is right side up.
  5. Jan 12, 2017 #4

    Ray Vickson

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    Better still: do not post an image at all; type out the problem (the much preferred PF way).
  6. Jan 14, 2017 #5
  7. Jan 14, 2017 #6
    Got it, thank you Teeth
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