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Homework Help: Sum of complex series

  1. Mar 8, 2010 #1
    1. The problem statement, all variables and given/known data

    Given that Wn = 3-n cos 2nӨ for n = 1, 2, 3, …, use de Moivre’s theorem to show that

    1 + W1 + W2 + W3 + … + WN-1 = [ 9 – 3 cos2Ө+ 3-N+1 cos2(N-1)Ө - 3-N+2 cos2NӨ] / [10 – 6cos2Ө]

    Hence show that the infinite series
    1 + W1 + W2 + W3 + …
    is convergent for all values of Ө, and find the sum to infinity

    Please i need help on how to solve this above question. though I have posted it before in my blog but was deleted. I don't know the reason for the deletion. I guessed I should have posted it in homework section

    2. Relevant equations

    3. The attempt at a solution
    I don't have a clue to this question, I have tried to use A.P and G.P formulas but proved difficult. I need help in order to teach my students preparing for external exams in further maths.
  2. jcsd
  3. Mar 8, 2010 #2


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    Staff Emeritus
    Science Advisor
    Gold Member

    Wn will be the real part of [tex]3^{-n}e^{2n\theta i}[/tex]. So summing up Wns is the same thing as taking the real part of a geometric series of terms like that.
  4. Mar 9, 2010 #3
    I have used ur suggestion,as follows common ration = 3-1 e2Өni, first term = 1, however I got stucked.
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