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Math Help - Double angle trig identity

  1. #1
    Member pberardi's Avatar
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    Double angle trig identity

    My teacher wrote this on the board
    cos^2(2x) = (1+cos(6x))/2
    Can someone please help me to understand this? I actually think I copied something down wrong. Thanks.
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    MHF Contributor Reckoner's Avatar
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    Quote Originally Posted by pberardi View Post
    My teacher wrote this on the board
    cos^2(2x) = (1+cos(6x))/2
    Can someone please help me to understand this? I actually think I copied something down wrong. Thanks.
    The power reduction formula for cosine is

    \cos^2x=\frac{1+\cos2x}2.

    So, \cos^22x=\frac{1+\cos4x}2.
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  3. #3
    Member pberardi's Avatar
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    So if I know for sure that the right hand side is (1+cos(6x))/2 then the left hand side would be cos^2(3x) ?? I am trying to figure out what exactly I copied down wrong. Thanks.
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    Quote Originally Posted by pberardi View Post
    So if I know for sure that the right hand side is (1+cos(6x))/2 then the left hand side would be cos^2(3x) ??
    Yes.
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