Question

Sin^4x-cos^4x= -cos2x or cos2x??

Answer 1

`- cos 2x`

Explanation:

`sin^4 x - cos^4 x = (sin^2 x + cos^2 x)(sin^2 x - cos^2 x)`

`= 1(sin^2 x - cos^2 x) = - (cos^2 x - sin^2 x) = - cos 2x`