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Math Help - verifying identities

  1. #1
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    verifying identities

    hey
    im new to the site but i have some ap calc summer work due tomorrow and i am STUCK with this trig identity. any ideas how to start it off?



    (cos^4)x = 1/8 (3+4cos2x+cos4x)
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  2. #2
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    Quote Originally Posted by still_nights View Post
    hey
    im new to the site but i have some ap calc summer work due tomorrow and i am STUCK with this trig identity. any ideas how to start it off?



    (cos^4)x = 1/8 (3+4cos2x+cos4x)
    Start with the left hand side:

    (\cos^2 x)^2 = \left( \frac{\cos (2x) + 1}{2}\right)^2

    using the well known double angle formula \cos (2x) = 2 \cos^2 (x) - 1.

    Now expand. Then use the double angle formula again.
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