Ta có :

a.    $x^{2}-3=x^{2}-(\sqrt{3})^{2}=(x-\sqrt{3})(x+\sqrt{3})$

b.   $x^{2}-6=x^{2}-(\sqrt{6})^{2}=(x+\sqrt{6})(x-\sqrt{6})$

c.   $x^{2}+2\sqrt{3}x+3=x^{2}+2.x.\sqrt{3}+(\sqrt{3})^{2}=(x+\sqrt{3})^{2}$

d.   $x^{2}-2\sqrt{5}x+5=x^{2}-2.x.\sqrt{5}+(\sqrt{5})^{2}=(x-\sqrt{5})^{2}$