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    is cos x= 1-sin^2 (x/2) or cos(x)=1-2sin^2 (x/2)
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    (Original post by jeja12)
    is cos x= 1-sin^2 (x/2) or cos(x)=1-2sin^2 (x/2)
    The latter.

    cos x = 1 - 2sin^2 (x/2)
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    Thank you, looks like I had it wrong in my notes after all
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    I tend to remember them the other way round, easier that way:

    cos2x = 1/2 + 1/2cos2x

    sin2x = 1/2 - 1/2cos2x
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    (Original post by jeja12)
    Thank you, looks like I had it wrong in my notes after all
    The easiest one to remember is cos 2x = cos^2 x - sin^2 x

    and then you can derive the others from that by replacing cos^2 x with 1 - sin^2 x or sin^2 x with 1 - cos^2 x.
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    Could you confirm my notes are correct since im really confused by the sin(3x) and the proof
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    (Original post by jeja12)
    Could you confirm my notes are correct since im really confused by the sin(3x) and the proof
    They are correct.
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    SinX= 2Sin[X/2]Cos[X/2]
 
 
 
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