Đáp án+Giải thích các bước giải:
`4) \frac{2 - \sqrt{5}}{2 + \sqrt{5}} + \frac{\sqrt{5} + 2}{\sqrt{5} - 2}`
`= \frac{2 - \sqrt{5}}{2 + \sqrt{5}} - \frac{2 + \sqrt{5}}{2 - \sqrt{5}}`
`= \frac{(2 - \sqrt{5})² - (2 + \sqrt{5})²}{(2 + \sqrt{5})(2 - \sqrt{5})}`
`= \frac{4 - 4\sqrt{5} + 5 - 4 - 4\sqrt{5} - 5}{4 - 5}`
`= \frac{-8\sqrt{5}}{-1}`
`= 8\sqrt{5}`
`5) \frac{6 - \sqrt{6}}{1 - \sqrt{6}} + \frac{6 - \sqrt{6}}{\sqrt{6}}`
`= - \frac{6 - \sqrt{6}}{\sqrt{6} - 1} + \frac{6 - \sqrt{6}}{\sqrt{6}}`
`= - \frac{\sqrt{6}(\sqrt{6} - 1)}{\sqrt{6} - 1} + \frac{\sqrt{6}(\sqrt{6} - 1)}{\sqrt{6}}`
`= - \sqrt{6} + \sqrt{6} - 1`
`= -1`
`6) \frac{1}{2 - \sqrt{3}} + \frac{1}{\sqrt{3} + 2}`
`= \frac{2 + \sqrt{3} + 2 - \sqrt{3}}{(2 - \sqrt{3})(2 + \sqrt{3})}`
`= \frac{4}{4 - 3}`
`= \frac{4}{1}`
`= 4`
