\(y=\left(cos^2x+sin^2x\right)\left(cos^2x-sin^2x\right).sinx.cosx\)
\(=\left(cos^2x-sin^2x\right).\dfrac{1}{2}\left(2sinx.cosx\right)=\dfrac{1}{2}cos2x.sin2x\)
\(=\dfrac{1}{4}sin4x\)
Do \(-1\le sin4x\le1\Rightarrow-\dfrac{1}{4}\le y\le\dfrac{1}{4}\)
\(y_{min}=-\dfrac{1}{4}\) khi \(sin4x=1\)
\(y_{max}=\dfrac{1}{4}\) khi \(sin4x=1\)