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Math Help - Stuck with an identity

  1. #1
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    Stuck with an identity

    Hi
    I am asked to tackle this one below
    I dont want to multiply the cosx by the cos2x yet all other routes I hit a dead end.
    Any help would be great

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  2. #2
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    Quote Originally Posted by 200001 View Post
    Hi
    I am asked to tackle this one below
    I dont want to multiply the cosx by the cos2x yet all other routes I hit a dead end.
    Any help would be great

    \cos x(\frac{\cos^2 x-\sin^2 x}{\cos x+\sin x})=\frac{1}{2}

    \cos x (\frac{(\cos x+\sin x)(\cos x-\sin x)}{\cos x+\sin x})=\frac{1}{2}

    Some simplication ,

    2\cos ^2 x-1-2\cos x\sin x=0

     <br />
\cos 2x=\sin 2x<br />
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  3. #3
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    brilliant, thanks
    I made the error of trying to keep cos2a in its 'nice form' in the numerator
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