$a)(4x+2)(x^2-2x+2)=0$
$\to 2(2x+1)(x^2-2x+2)=0$
$\text{Do:}$
$x^2-2x+2=(x-1)^2+1$
$\to x^2-2x+2 \ne 0$
$\to 2x+1=0$
$\to x=-\dfrac{1}{2}$
$\text{Vậy S=}-\dfrac{1}{2}$
$b)x^4-5x^2+6=0$
$\to x^4-2x^2-3x^2+6=0$
$\to x^2(x^2-2)-3(x^2-2)=0$
$\to (x^2-2)(x^2-3)=0$
$\to$
$x^2-2=0$
$\to x=\pm \sqrt{2}$
$\to$
$x^2-3=0$
$\to x= \pm \sqrt{3}$
$\text{Vậy S=} \pm \sqrt{2}; \pm \sqrt{3}$
