trig verify the identify

**algebra2** · April 17th 2008, 05:46 PM

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

**topsquark** · April 17th 2008, 06:12 PM

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

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