1. ## Prove this identity

Can anyone help me prove this trig identity:

Cos(4ά)+4cos(2ά)+3=8cos(4ά)

Thanks

2. I think the identity is

$\cos(4\alpha)+4\cos(2\alpha)+3=8\cos^{4}(\alpha),$ right?

3. Yes, thats right. Sorry...

Thanks

4. Ok. So what ideas have you had so far?

5. [IMG]URL=http://img263.imageshack.us/i/64044892.png/]

Thats what I did...I try everything...still nothing

1. $\cos(4\alpha)=\cos^{4}(\alpha)-2\sin^{2}(\alpha)\cos^{2}(\alpha)+\sin^{4}(\alpha)-4\sin^{2}(\alpha)\cos^{2}(\alpha).$
2. In your expression, you still have a $\cos(2\alpha)$ in there. Reduce the angle, and see what you get.