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aj026 · May 6, 2009, 08:09 AM

cos²Θ = cos²Θ – sin ²Θ

Unknown008 · May 6, 2009, 08:23 AM

Are you sure this is the right identity?

I'm pretty sure you meant

$cos2\theta = cos^2\theta - sin^2 \theta$

then, on the left hand side;

$cos2\theta = cos(\theta + \theta)$

$cos(\theta + \theta) = cos\theta cos\theta - sin\theta sin\theta$

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

Note, you make use of the identity

$cos(A + B) = cosA cosB - sinA sinB$

liang11 · Jul 26, 2009, 03:24 AM

Prove (Sin5A - Sin4A)/(Cos4A + Cos5A) = Tan A/2

DESPERATE NEED OF HELP!

galactus · Jul 26, 2009, 04:38 AM

Originally Posted by liang11

Prove (Sin5A - Sin4A)/(Cos4A + Cos5A) = Tan A/2

DESPERATE NEED OF HELP!!

Start your own thread instead of using an old one. You are more apt to get help that way.