a) `M=(\sqrt{a}+\sqrt{b})/(\sqrt{a}-\sqrt{b})-(4\sqrt{ab})/(a-b)+\sqrt{b}/(\sqrt{a}+\sqrt{b})`
`=(\sqrt{a}+\sqrt{b})/(\sqrt{a}-\sqrt{b})-(4\sqrt{ab})/((\sqrt{a}-\sqrt{b})(\sqrt{a}+\sqrt{b}))+\sqrt{b}/(\sqrt{a}+\sqrt{b})`
`=((\sqrt{a}+\sqrt{b})^{2}-4\sqrt{ab}+\sqrt{b}(\sqrt{a}-\sqrt{b}))/((\sqrt{a}-\sqrt{b})(\sqrt{a}+\sqrt{b}))`
`=(a+2\sqrt{ab}+b-4\sqrt{ab}+\sqrt{ab)-b}/((\sqrt{a}-\sqrt{b})(\sqrt{a}+\sqrt{b}))`
`=(a-\sqrt{ab})/((\sqrt{a}-\sqrt{b})(\sqrt{a}+\sqrt{b}))`
`=(\sqrt{a}(\sqrt{a}-\sqrt{b}))/((\sqrt{a}-\sqrt{b})(\sqrt{a}+\sqrt{b}))` `=\sqrt{a}/(\sqrt{a}+\sqrt{b})`
b) Khi `a=3 ; b=4-2\sqrt{3}`
`->\sqrt{3}/(\sqrt{3}+\sqrt{4-2\sqrt{3}})`
`->\sqrt{3}/(\sqrt{3}+\sqrt{(\sqrt{3}-1)^{2}}`
`->\sqrt{3}/(\sqrt{3}+|\sqrt{3}-1|)`
`->\sqrt{3}/(2\sqrt{3}-1)`
Vậy `a=3 ; b=4-2\sqrt{3}` thì `M=\sqrt{3}/(2\sqrt{3}-1)`
