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    Its question 9 of january 2009 C3 for OCR.

    9i) By first expanding cos(2X+X) prove that
    cos3X = 4cos^3 X - 3cosx

    (The x is actually theta in the question)

    So I put it into the addition fornular to get cos2x.cosx - sin2x.sinx

    Then after this I get stuck. I'm presuming you use the identities such as

    (cos2x = 2cos^2x-1)
    &
    (sin2x = 2sinxcosx)

    But it doesn't get me anywhere thanks
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    (Original post by Pistolopete)
    Then after this I get stuck. I'm presuming you use the identities such as

    (cos2x = 2cos^2x-1)
    &
    (sin2x = 2sinxcosx)

    But it doesn't get me anywhere thanks
    Those will do, and you're then only one or two steps away from the solution; have a go.
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    And sin²x+cos²x=1 then you're good to go
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    (Original post by Pistolopete)
    x
    cos(2x+x)=
    cos2xcosx-sin2xsinx=
    (2cos^2x-1)cosx-(2sinxcosx)sinx=
    2cos^3x-cosx-2sin^2xcosx=
    2cos^3x-cosx-2(1-cos^2x)cosx=
    2cos^3x-cosx-2cosx+2cos^3x=
    4cos^3x-3cosx



    Edit: what's with the neg rep?
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    Thankyou for the responses they all helped
 
 
 
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Updated: December 21, 2010
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