$4\cos^2x-2(\sqrt3-\sqrt2)\cos x-\sqrt6=0$
$\Delta'=(\sqrt3-\sqrt2)^2+4\sqrt6=5+2\sqrt6=(\sqrt3+\sqrt2)^2>0$
$\to \sqrt{\Delta'}=\sqrt3+\sqrt2$
PT có hai nghiệm phân biệt:
$\cos x=\dfrac{\sqrt3-\sqrt2+\sqrt3+\sqrt2}{4}=\dfrac{\sqrt3}{2}$
$\to x=\pm\dfrac{\pi}{6}+k2\pi$
$\cos x=\dfrac{\sqrt3-\sqrt2-\sqrt3-\sqrt2}{4}=\dfrac{-\sqrt2}{2}$
$\to x=\pm\dfrac{3\pi}{4}+k2\pi$
