# Math Help - Proving trigonometry equation

1. ## 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

2. Originally Posted by greghunter
Prove that $\cos^4x- \sin^4x\equiv\cos^2x-sin^2x$
$a^4 - b^4=(a^2+b^2)(a^2-b^2)$

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