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Math Help - Trig identity?

  1. #1
    Junior Member
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    Trig identity?

    I'm having trouble trying to figure out how:

    \cos^22\theta

    turns into:

    1/2 (1 + cos4\theta)

    Are we using some kind of trig identity?

    Thanks!
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  2. #2
    MHF Contributor Also sprach Zarathustra's Avatar
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    Yes:

    cos^2(t)=1/2{1+cos(2t)}
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  3. #3
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    How do we get the cos4(t)?
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  4. #4
    Senior Member eumyang's Avatar
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    Not sure what you're asking. The identity Also sprach Zarathustra mentioned is the power reduction identity.
    \cos^2 \,x = \dfrac{1}{2}(1 + \cos \,2x)

    Since you're looking for \cos^2\,2\theta, just plug in 2\theta for x.
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