Please help me out with this question

Use the expansion of cos(A+B) to show that

Cos3x=4(cos^3)x-3cosx

This is what i did:

Cos 3x = (cos^2)x-(sin^2)x

= (3cos^2)x-1

I think i am doing something wrong..

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- Nov 15th 2010, 09:59 AMmike789Prove Cos3x=4(cos^3)x-3cosx
Please help me out with this question

Use the expansion of cos(A+B) to show that

Cos3x=4(cos^3)x-3cosx

This is what i did:

Cos 3x = (cos^2)x-(sin^2)x

= (3cos^2)x-1

I think i am doing something wrong.. - Nov 15th 2010, 10:21 AMUnknown008
Well, you actually did something wrong...

Use the expansion of cos(A+B) as you were told.

Now, you have cos(3x), make it cos(2x + x) and use the expansion. What do you get? - Nov 15th 2010, 10:34 AMmike789
- Nov 15th 2010, 10:37 AMUnknown008
I'm sorry, what do you mean by addition formulae?

- Nov 15th 2010, 10:48 AMmike789
- Nov 15th 2010, 10:50 AMUnknown008
Um... is that not what I told you to use?

- Nov 15th 2010, 11:02 AMmike789
- Nov 15th 2010, 11:04 AMUnknown008
Yes, but how about using it for the time being to see where it leads us? (Smile)

- Nov 15th 2010, 11:57 AMe^(i*pi)