Ta có :
a. $x^{2}-5=0$
<=> $x^{2}-(\sqrt{5})^{2}=0$
<=> $(x+\sqrt{5})(x-\sqrt{5})=0$
<=> $\left\{\begin{matrix}x+\sqrt{5}=0 & \\ x-\sqrt{5}=0 & \end{matrix}\right.$
<=> $\left\{\begin{matrix}x=\sqrt{5}& \\ x=-\sqrt{5} & \end{matrix}\right.$
Vậy $S={-\sqrt{5},\sqrt{5}}$
b. $x^{2}-2\sqrt{11}x+11=0$
<=> $x^{2}-2.x.\sqrt{11}+(\sqrt{11})^{2}=0$
<=> $(x-\sqrt{11})^{2}=0$
<=> $x-\sqrt{11}=0$
<=> $x=\sqrt{11}$
Vậy $S={\sqrt{11}}$ .
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