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    By expressing
    cos3a = 2a+a , show that ,
    cos3a=4cos^3a-cosa

    Im really stuck
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    (Original post by Semaphore)
    By expressing
    cos3a = 2a+a , show that ,
    cos3a=4cos^3a-cosa

    Im really stuck
    Try using cosine formulae.
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    (Original post by Azhar Rana)
    Try using cos(a+b).
    So
    cos(2a+a) = 4cos^3a - cosa

    What to do next?:/
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    replace cos(2α + α) with cos(2α)cos(α) – sin(2α)sin(α).
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    Try that, however i do not want to tell you all of the information but obviously you've already done 1/3 of the work in the way you expressed cos(3x) as cos(2x+x)
    Also note the formula for

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    (Original post by Semaphore)
    So
    cos(2a+a) = 4cos^3a - cosa

    What to do next?:/
    That's what you're supposed to be showing!

    If you know the formula for cos(A + B) then put A = 2a and B = a into the formula and then rearrange using a standard identity that links cos^2 x and sin^2 x
 
 
 
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