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    The answer is cos2x. Why?
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    (Original post by superkillball)
    The answer is cos2x. Why?
    cos^4 - sin^4 = (cos^2 - sin^2)(cos^2 + sin^2)

    cos^2 + sin^2 = 1, so

    cos^4 - sin^4 = cos^2 - sin^2 = cos2x
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    Maybe something to do with difference of two squares?
    I'm not sure.
    What module is it?
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    Ah well done James, I shouldn have seen that.
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    (Original post by JamesF)
    cos^4 - sin^4 = (cos^2 - sin^2)(cos^2 + sin^2)

    cos^2 + sin^2 = 1, so

    cos^4 - sin^4 = cos^2 - sin^2 = cos2x
    thanks a lot
 
 
 
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