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Math Help - Simplifying trig problem

  1. #1
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    Dec 2009
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    Simplifying trig problem

    Hi guys, I need help on simplifying this trig equation, any help appreciated

    I have to show that

    cos((n+1).X) = 2cosn X .cosX - cos((n-1).X)

    But I do not know what steps to take.

    EDIT: I think this is in the wrong forum. sorry.
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  2. #2
    MHF Contributor red_dog's Avatar
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    Medgidia, Romania
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    Use the identity \cos a+\cos b=2\cos\frac{a+b}{2}\cos\frac{a-b}{2}

    for \cos(n+1)x+\cos(n-1)x
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