1. proving indentities..

i cant quite seem to get this.. thanks for the help. x

Prove the following indenty. You may assume the formulas for sun2x and cos2x.

cos3x = 4cos(^3)x - 3cosx

2. Originally Posted by pop_91
i cant quite seem to get this.. thanks for the help. x

Prove the following indenty. You may assume the formulas for sun2x and cos2x.

cos3x = 4cos(^3)x - 3cosx
cos (3x)=cos(x+2x)
use identity
cos (A+B)=cos B cos B-sin A sin B
cos 2A an sin 2A.
$\sin^2 x=1- \cos^2 x$

3. i've done that and sub in 1-cos^2x but i dont get the correct answer

4. Originally Posted by pop_91
i've done that and sub in 1-cos^2x but i dont get the correct answer
$\cos (3x)=\cos(x+2x)=\cos x \cos 2x-\sin x \sin 2x$
$=\cos x ( 2 \cos^2 x -1) - \sin x (2 \sin x \cos x)$
$=2 \cos^3 x -\cos x - 2 \sin^2 x \cos x$

put $\sin^2 x=1- \cos^2 x$

5. ive got it now! thanks