`a)` `\sqrt{8-2\sqrt{7}}`
`=\sqrt{7-2\sqrt{7.1}+1}`
`=\sqrt{(\sqrt{7}-1)^2}`
`=|\sqrt{7}-1|`
`=\sqrt{7}-1` (do `\sqrt{7}-1>0)`
`b)` `\sqrt{5-2\sqrt{6}}`
`=\sqrt{2-2\sqrt{2.3}+3}`
`=\sqrt{(\sqrt{2}-\sqrt{3})^2}`
`=|\sqrt{2}-\sqrt{3}|`
`=\sqrt{3}-\sqrt{2}` (do `\sqrt{2}-\sqrt{3}<0)`