Prove that $\displaystyle \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

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- Jun 27th 2009, 03:23 AMgreghunterProving trigonometry equation
Prove that $\displaystyle \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, 03:45 AMPlato
- Jun 27th 2009, 04:18 AMgreghunter