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Double angle formulae question watch

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    In my edexcel text book it says The double angle formulae can then be used to find expressions for other multiples e.g. sin3A or cos3A
    The examples it gives are
    sin3A = 3sinA - 4(sinA)^3
    cos 3A = 4(cosA)^3 - 3cosA
    It also talks about Ex3D q11 which says you have to use sin (2A + A) to show the equation above is true.
    I am competely baffled! :confused: Does anyone please know how to do this???
    Thanks in advance!
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    Don't worry! Just worked it out
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    Wow, lol. Never thought of doing it that way. Here's how I did it:

    Sin3a = Sin(2a+a)
    =Sin2aCosa+Cos2aSina
    =(2SinaCosa)Cosa+(1-2Sin²a)Sina
    =2SinaCos²a +Sina - 2Sin³a
    =2Sina(1-Sin²a) + Sina - 2Sin³a
    =2Sina -2Sin³a + Sina - 2Sin³a
    =3Sina - 4Sin³a.

    edit - Oh. ^^;;
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    Thanks anyway Rabite!
 
 
 
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Updated: June 12, 2006

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