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Homework Help: Summation equation help

  1. Feb 14, 2006 #1
    I am to show that...

    [tex]\sum_{n=-N}^{+N} cos(\alpha -nx)=cos\alpha \frac{sin(N+0.5)x}{sin(x/2)}[/tex]

    [tex]\sum_{n=-N}^{+N} cos(\alpha)cos(nx)+\sum_{n=-N}^+Nsin(\alpha)\frac{sin(N+0.5)x}{sin(x/2)}[/tex]

    [tex]\sum_{n=-N}^{+N}sin(\alpha)\frac{sin(N+0.5)x}{sin(x/2)} =0[/tex]

    [tex]cos(\alpha) 2 \sum_{n=0}^{+N} cos(nx)[/tex]

    I know of a rule that shows...

    [tex]\frac{1}{2}+cos(x)+cos(2n)+...cos(nx)=\frac{sin(N+0.5)x}{2sin(x/2)}[/tex]

    but I dont see how to apply it to get my answer, since my summation is similar to equation (9) on this site: http://mathworld.wolfram.com/Cosine.html

    any ideas?
     
  2. jcsd
  3. Feb 14, 2006 #2

    Hurkyl

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    I think you've messed up in stating your problem -- it doesn't really make sense.

    Anyways, you know trig identities, right? You could try applying some of them.

    Or, you could always grind through an inductive proof.
     
  4. Feb 15, 2006 #3
    oh wow, i totally messed up there...

    [tex]\sum_{n=-N}^{+N} cos(\alpha -nx)=cos\alpha \frac{sin(N+0.5)x}{sin(x/2)}[/tex]

    [tex]cos(\alpha -nx) =cos(x)cos(nx)+sin(\alpha)sin(nx)[/tex]

    [tex]\sum_{n=-N}^{+N} cos(\alpha)cos(nx)+\sum_{n=-N}^{+N} sin(\alpha)sin(nx)[/tex]

    [tex]sin(\alpha) \sum_{n=-N}^{+N} sin(nx)=0[/tex] since it is an odd function

    so I am left with...
    [tex]cos(\alpha) 2 \sum_{n=0}^{+N} cos(nx)=cos\alpha \frac{sin(N+0.5)x}{sin(x/2)}[/tex]
    [tex]2 \sum_{n=0}^{+N} cos(nx)=\frac{sin(N+0.5)x}{sin(x/2)}[/tex]

    there is a rule that shows:
    [tex]\frac{1}{2}+cos(x)+cos(2n)+...cos(nx)=\frac{sin(N+ 0.5)x}{2sin(x/2)}[/tex]

    I am stuck on this part and I dont know where to go from here.
     
  5. Feb 15, 2006 #4

    Hurkyl

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    Can you write that rule in summation notation?
     
  6. Feb 15, 2006 #5

    VietDao29

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    This is wrong, counter-example: N = 0, the LHS is 2, whereas the RHS is 1, and it's true that: [tex]2 \neq 1[/tex], right?
    You are wrong when assuming that:
    [tex]\cos \alpha \sum_{n = -N} ^ {+N} (\cos (nx)) = 2 \cos \alpha \sum_{n = 0} ^ {+N} (\cos (nx))[/tex]
    In fact, it should read:
    [tex]\cos \alpha \sum_{n = -N} ^ {+N} (\cos (nx)) = 2 \cos \alpha \sum_{n = 1} ^ {+N} (\cos (nx)) + \cos \alpha \cos (0n) = 2 \cos \alpha \sum_{n = 1} ^ {+N} (\cos (nx)) + \cos \alpha[/tex]
    [tex]= \cos \alpha \left( 1 + 2 \left( \sum_{n = 1} ^ {+N} \cos (nx) \right) \right)[/tex].
    Now you can go from here, right? Hint: follow Hurkyl's suggestion. :)
     
  7. Feb 15, 2006 #6
    [tex]\frac{sin(N+ 0.5)x}{2sin(x/2)}=-\frac{1}{2}+\sum_{n = 1} ^ {+N} \cos (nx) [/tex]

    so now everything fits into place!

    Just out of curiosity, how would someone derive: [tex]\frac{1}{2}+cos(x)+cos(2n)+...cos(nx)=\frac{sin(N+ 0.5)x}{2sin(x/2)}[/tex]?
     
  8. Feb 15, 2006 #7

    shmoe

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    Use [tex]\cos(z)=\frac{e^{iz}+e^{-iz}}{2}[/tex] to turn it into a geometric sum.
     
  9. Feb 15, 2006 #8

    Hurkyl

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    Induction works too.
     
  10. Feb 15, 2006 #9
    would the answer include the imaginary part?

    i've found an example in mathworld but it isnt the sum from 1 to infinity, but from 0 to infinity: http://mathworld.wolfram.com/Cosine.html


    how is it possible that their cosine on the numerator is cancelled out to form [tex]\frac{sin(N+0.5)x}{sin(x/2)}[/tex] when the summation is changed from (0 to inifinity is what they have) to 1 to infinity?
     
  11. Feb 15, 2006 #10

    shmoe

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    There won't be an imaginary part.

    The sum on mathworld, (9)-(13) I guess you mean, is a little different from yours (note it doesn't go to infinity). You both have a term for n=0, but yours is half theirs and your argments for the sin in the numerator are different.

    Their method of derivation is essentially what I suggested (though they should have said something about the x=integer multiple of 2*pi case). You might want to work with yours in the form [tex]\sum_{n = -N} ^ {+N} \cos (nx)[/tex] though.
     
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