if y=cos^2x-sin^2x, then y'=

Answer 1

y = cos^2 x - sin^2 x
then
y' = - 2 cos x sin x - 2 sin x cos x
= -4 sin x cos x

Answer 2

by the way that is -2 sin (2x)

Answer 3

or
y = cos^2 x - sin^2 x
= cos (2x)
dy/dx = -2sin(2x) , which is Damon's last answer.