Bài 1
1) [tex]A=\frac{1}{2}\sqrt{20}- \sqrt{80}+\frac{2}{3}\sqrt{45}
= \frac{1}{2}.2\sqrt{5}-4\sqrt{5}+ \frac{2}{3}.3\sqrt{5}
=\sqrt{5}-4\sqrt{5}+ 2\sqrt{5}
=-\sqrt{5}[/tex]
Vậy A=-\sqrt{5}[/tex]
2) [tex]B= (2+ \frac{5-\sqrt{5}}{\sqrt{5}-1}). (2-\frac{5+\sqrt{5}}{\sqrt{5}+1})
=(2+\frac{\sqrt{5}.(\sqrt{5}-1)}{\sqrt{5}-1}).(2-\frac{\sqrt{5}.(\sqrt{5}+1)}{\sqrt{5}+1})
= (2+\sqrt{5}).(2-\sqrt{5}) =2^{2}-(\sqrt{5})^{2}
= 4-5
=-1[/tex]