# Math Help - trig verify the identify

1. ## trig verify the identify

verify the identify:

since cos2u=cos^2-sin^2

would the same be applied to cos3u..as in = cos^3-sin^3, or am i wrong

2. Originally Posted by algebra2
verify the identify:

since cos2u=cos^2-sin^2

would the same be applied to cos3u..as in = cos^3-sin^3, or am i wrong
$cos(A + B) = cos(A)~cos(B) - sin(A)~sin(B)$

Soooo....
$cos(3 \theta) = cos(2\theta + \theta)$

$= cos(2\theta)~cos(\theta) - sin(2\theta)~sin(\theta)$

and you can take it from here.

-Dan