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# Need help with simple double angles

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

cos2x = 1/2 + 1/2cos2x

sin2x = 1/2 - 1/2cos2x
5. (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.
6. Could you confirm my notes are correct since im really confused by the sin(3x) and the proof
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7. (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.
8. SinX= 2Sin[X/2]Cos[X/2]

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