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Math Help - proving indentities..

  1. #1
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    Post proving indentities..

    i cant quite seem to get this.. thanks for the help. x


    Prove the following indenty. You may assume the formulas for sun2x and cos2x.

    cos3x = 4cos(^3)x - 3cosx
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  2. #2
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    Quote Originally Posted by pop_91 View Post
    i cant quite seem to get this.. thanks for the help. x


    Prove the following indenty. You may assume the formulas for sun2x and cos2x.

    cos3x = 4cos(^3)x - 3cosx
    cos (3x)=cos(x+2x)
    use identity
    cos (A+B)=cos B cos B-sin A sin B
    cos 2A an sin 2A.
     \sin^2 x=1- \cos^2 x
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  3. #3
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    i've done that and sub in 1-cos^2x but i dont get the correct answer
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  4. #4
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    Quote Originally Posted by pop_91 View Post
    i've done that and sub in 1-cos^2x but i dont get the correct answer
    \cos (3x)=\cos(x+2x)=\cos x \cos 2x-\sin x \sin 2x
    =\cos x ( 2 \cos^2 x -1) - \sin x (2 \sin x \cos x)
    =2 \cos^3 x -\cos x  - 2 \sin^2 x \cos x

    put  \sin^2 x=1- \cos^2 x
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  5. #5
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    ive got it now! thanks
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