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# Double angle formulae question watch

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1. 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! Does anyone please know how to do this???
2. Don't worry! Just worked it out
3. 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. ^^;;
4. Thanks anyway Rabite!

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Updated: June 12, 2006
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