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Verify infinite series

  • Thread starter jj1986
  • Start date
  • #1
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Homework Statement



Show that for all integers n [tex]\geq[/tex] 1,
cos(2x) + cos(4x) + ... + cos(2nx) = [tex]\frac{1}{2}[/tex] ([tex]\frac{sin((2n+1)x)}{sin(x)}[/tex]-1)

Use this to verify that
[tex]\sum_{n=1}^{\infty}(\int_{0}^{\pi}[/tex] x([tex]\pi[/tex]-x)cos(2nx)dx) =

[tex]\frac{-1}{2}\int_{0}^{\pi}[/tex] x([tex]\pi[/tex]-x)dx)


Homework Equations





The Attempt at a Solution


I proved the first part of this problem using induction, however I don't see how I can use that to verify the second part. Maybe I can bring the summation into the integral and get the sum of cos(2nx), but I still don't see how that would give me what I need to prove. Any suggestions?
 

Answers and Replies

  • #2
CompuChip
Science Advisor
Homework Helper
4,302
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If you interchange the sum with the integral, you can substitute the identity you have proved.
Then the second part (with the - 1) will give you the answer you want, you will just need to prove that
[tex]\lim_{n \to \infty} \frac12 \int_0^\pi x (\pi - x) \frac{\sin((2n+1)x)}{\sin(x)}\, dx = 0[/tex]
 

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