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Double angle formulae question

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!
Reply 1
Don't worry! Just worked it out :biggrin:
Reply 2
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. ^^;;
Reply 3
Thanks anyway Rabite! :smile: