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Math Help - Another Identity problem

  1. #1
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    Another Identity problem

    Hi can anyone help me with this?


    Thanks.


    Prove the identity:

    cos3\theta = 4cos^3\theta - 3cos\theta

    [HINT: Express cos3\theta as cos (2\theta + \theta]
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  2. #2
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  3. #3
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    Thanks raimee, your a legend.
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  4. #4
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    Ok i've got it down to:

    2cos^3\theta - cos\theta - 2sin^2\theta cos\theta

    Im trying to use the identity of sin^2\theta = 1 - cos^2\theta

    But im unsure how to use the identity as is is 2sin^2\theta

    Thanks again
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  5. #5
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    Quote Originally Posted by anothernewbie View Post
    Ok i've got it down to:

    2cos^3\theta - cos\theta - 2sin^2\theta cos\theta

    Im trying to use the identity of sin^2\theta = 1 - cos^2\theta

    But im unsure how to use the identity as is is 2sin^2\theta

    Thanks again
     <br />
2\sin^2 \theta=2(1 - cos^2\theta)=2-2\cos^2 \theta<br />
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  6. #6
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    That was too obvious

    Thanks
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