Ptrinh tương đương vs
$(\sin^2x + \cos^2x)^2 - 2\sin^2x \cos^2 x = \dfrac{1}{2}$
$<-> \sin^2x \cos^2x = \dfrac{1}{4}$
$<-> \dfrac{1}{4} \sin^2(2x) = \dfrac{1}{4}$
$<-> \sin^2(2x) = 1$
Vậy $\sin(2x) = 1$ hoặc $\sin(2x) = -1$ hay $x = \dfrac{\pi}{4} + k\pi$ hoặc $x = -\dfrac{\pi}{4} + k\pi$.