# Proving trigonometry equation

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• Jun 27th 2009, 04:23 AM
greghunter
Proving trigonometry equation
Prove that $\cos^4x- \sin^4x\equiv\cos^2x-sin^2x$

I have the trig rules but don't know what to do with them.

Any help would be much appreciated
• Jun 27th 2009, 04:45 AM
Plato
Quote:

Originally Posted by greghunter
Prove that $\cos^4x- \sin^4x\equiv\cos^2x-sin^2x$

$a^4 - b^4=(a^2+b^2)(a^2-b^2)$
• Jun 27th 2009, 05:18 AM
greghunter
Quote:

Originally Posted by Plato
$a^4 - b^4=(a^2+b^2)(a^2-b^2)$

Thanks, that's really helpful