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Math Help - General solution

  1. #1
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    General solution

    Find the general solution, in radians, of the equation

    cos2a + cos4a + cos6a = 0

    I have this equation to play with:

    a = 2πn (+or-) α

    but i dont know what to do with it :S
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  2. #2
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    Quote Originally Posted by djmccabie View Post
    Find the general solution, in radians, of the equation

    cos2a + cos4a + cos6a = 0

    I have this equation to play with:

    a = 2πn (+or-) α

    but i dont know what to do with it :S
    Hint: \cos(2a) + \cos(6a)=2 \cos(2a) \cos (4a).
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  3. #3
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    Why is this true :S
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  4. #4
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    Quote Originally Posted by djmccabie View Post
    Why is this true :S
    in general we have this identity: \cos x + \cos y = 2 \cos \left(\frac{x+y}{2} \right) \cos \left(\frac{x-y}{2} \right). do you remember? now just put x=6a and y=2a and see what you get!
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  5. #5
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    Quote Originally Posted by NonCommAlg View Post
    in general we have this identity: \cos x + \cos y = 2 \cos \left(\frac{x+y}{2} \right) \cos \left(\frac{x-y}{2} \right). do you remember?
    No. I gotta teach myself a lot of the maths I haven't come across this thanks a lot!

    are there any more important ones i should be aware of? I have learned compound angle and double angle formulae
    Last edited by mr fantastic; May 8th 2009 at 02:07 PM. Reason: Merged posts
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