$y = \sin^4x + \cos^4x - \sin^2x - \cos^2x$
$= (\sin^2 + \cos^2x)^2 -2\sin^2x\cos^2x - (\sin^2x + \cos^2x)$
$= 1 - \dfrac{1}{2}\sin^22x - 1$
$= -\dfrac{1}{2}\sin^22x$
Ta có:
$0 \leq \sin^22x \leq 1$
$-\dfrac{1}{2} \leq -\dfrac{1}{2}\sin^22x \leq 0$
Hay $-\dfrac{1}{2} \leq y \leq 0$
Vậy $\min y = -\dfrac{1}{2} \Leftrightarrow \sin^22x = 1 \Leftrightarrow \sin2x = \pm 1 \Leftrightarrow x = \dfrac{\pi}{4} + k\dfrac{\pi}{2}$
$\max y = 0 \Leftrightarrow \sin2x = 0\Leftrightarrow x = k\dfrac{\pi}{2} \quad (k \in \Bbb Z)$
