Bài `2.`

`1.`

`A=\sqrt{4-\sqrt{7}}-\sqrt{4+\sqrt{7}}`

`=(\sqrt{8-2\sqrt{7}}-\sqrt{8+2\sqrt{7}})/(\sqrt{2}`

`=(\sqrt{(\sqrt{7}-1)^2} - \sqrt{(\sqrt{7}+1)^2})/(\sqrt{2}`

`=(\sqrt{7}-1-\sqrt{7}-1)/(\sqrt{2}`

`=(-2)/(\sqrt{2})`

`=-\sqrt{2}`

`3.`

`C=\sqrt{2-\sqrt{3}}.(\sqrt{6}+\sqrt{2})`

`=\sqrt{4-2\sqrt{3}}.(\sqrt{3}+1)`

`=\sqrt{(\sqrt{3}-1)^2} . (\sqrt{3}+1)`

`=(\sqrt{3}-1).(\sqrt{3}+1)`

`=3-1`

`=2`.