Đáp án:
$\displaystyle \begin{array}{{>{\displaystyle}l}} Bài\ 4:\ \\ a.\ \sqrt{2} ;\ b.\ 4;\ c.\ 2\sqrt{3} ;\ d.-\frac{\sqrt{3}}{2}\\ Bài\ 5:\\ a.\ -1\ b.\ 5;\ c.\ 44;\ d.\ 22 \end{array}$
Giải thích các bước giải:
$\displaystyle \begin{array}{{>{\displaystyle}l}} Bài\ 4:\\ a.\frac{\sqrt{18} -\sqrt{2}}{2} =\frac{\sqrt{2}( 3-1)}{2} =\sqrt{2}\\ b.\ \frac{\sqrt{108} -2\sqrt{3}}{\sqrt{3}} =\frac{6\sqrt{3} -2\sqrt{3}}{\sqrt{3}} =4\\ c.\ \frac{\sqrt{108} -2\sqrt{3}}{2} =\frac{6\sqrt{3} -2\sqrt{3}}{2} =2\sqrt{3}\\ d.\frac{\sqrt{27} -\sqrt{48}}{2} =\frac{\sqrt{3}( 3-4)}{2} =\frac{-\sqrt{3}}{2} \ \\ Bài\ 5:\\ a.\ \left(\sqrt{3} -2\right)\left(\sqrt{3} +2\right) =\left(\sqrt{3}\right)^{2} -2^{2} =3-4=-1\\ b.\ \left(\sqrt{6} -1\right)\left(\sqrt{6} +1\right) =\left(\sqrt{6}\right)^{2} -1^{2} =6-1=5\\ c.\left(\sqrt{5} +7\right) .\left( 7-\sqrt{5}\right) =7^{2} -\left(\sqrt{5}\right)^{2} =49-5=44\\ d.\ \ \left(\sqrt{3} +5\right)\left( 5-\sqrt{3}\right) =5^{2} -\left(\sqrt{3}\right)^{2} =25-3=22 \end{array}$