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Homework Help: Trigo Question - just for fun

  1. Jul 8, 2008 #1
    Alright, not really homework or coursework.

    Sum(from k = 0 to infinity) (cos thetha)^2k = 5
    So i subbed in the first value, 0 and took the one out.
    I have to find the value of cos(2thetha)


    and then there was a pattern, but i didn't really know what to do.

    Thanks

    P.S. Not really familiar with Latex yet.
     
  2. jcsd
  3. Jul 8, 2008 #2

    dynamicsolo

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    Homework Helper

    If you know the formula for the sum of an infinite series, you can work backwards. Substitute x = (cos theta)^2 to make your infinite sum

    sum (k=0 to infinity) x^k = 5 .

    What value of x works? That's your value for (cos theta)^2 . You can use a trig identity (or even a calculator) to find cos(2·theta)...
     
  4. Jul 8, 2008 #3

    HallsofIvy

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    Science Advisor

    [itex]\theta[/itex] is a fixed number. As dynamicsolo said, first use the formula for the sum of an infinite geometric sequence
    [itex]\sum_{k=0}^\infty x^k= \frac{1}{1- x}[/tex] to determine what x is. Then [itex]\theta= cos^{-1}/sup](x)[/itex].
     
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