1. ## Proof question

How can I prove 4(sin^4 x+ cos^4 x) always equals cos 4x + 3?

I don't think I've been taught the identities that I need...

2. Originally Posted by ctt
How can I prove 4(sin^4 x+ cos^4 x) always equals cos 4x + 3?

I don't think I've been taught the identities that I need...

$\cos(x)=\frac{e^{ix}+e^{-ix}}{2}$
$\sin(x)=\frac{e^{ix}-e^{-ix}}{2i}$