1. ## Another Identity problem

Hi can anyone help me with this?

Thanks.

Prove the identity:

$cos3\theta = 4cos^3\theta - 3cos\theta$

[HINT: Express $cos3\theta$ as $cos (2\theta + \theta$]

2. Thanks raimee, your a legend.

3. Ok i've got it down to:

$2cos^3\theta - cos\theta - 2sin^2\theta cos\theta$

Im trying to use the identity of $sin^2\theta = 1 - cos^2\theta$

But im unsure how to use the identity as is is $2sin^2\theta$

Thanks again

4. Originally Posted by anothernewbie
Ok i've got it down to:

$2cos^3\theta - cos\theta - 2sin^2\theta cos\theta$

Im trying to use the identity of $sin^2\theta = 1 - cos^2\theta$

But im unsure how to use the identity as is is $2sin^2\theta$

Thanks again
$
2\sin^2 \theta=2(1 - cos^2\theta)=2-2\cos^2 \theta
$

5. That was too obvious

Thanks