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Math Help - Prove Identity

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    Prove Identity

    Trig Identity Prove:

    cos(x+y)cos(x-y)=cos^2(x)+cos^2(y)-1
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  2. #2
    MHF Contributor Bruno J.'s Avatar
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    \cos(x+y)=\cos x \cos y - \sin x \sin y

    \cos(x-y)=\cos x \cos y + \sin x \sin y

    \cos(x+y)\cos(x-y)=(\cos x \cos y)^2 - (\sin x \sin y)^2

    \cos(x+y)\cos(x-y)=(\cos x \cos y)^2 - (1-\cos^2 x)(1-\cos^2y)\


    \cos(x+y)\cos(x-y)=\cos^2x+\cos^2y -1
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