Bài 1.
`a)` `\sqrt{72.50}`
`=\sqrt{6^2 . 2 .5^2 .2}`
`=\sqrt{(6 . 2.5)^2}=|6.2.5|=60`
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`b)` `\sqrt{12,8.\ 1,8}`
`=\sqrt{64 .\ 0,2 .\ 9 .\ 0,2}`
`=\sqrt{(8.\ 3 . \ 0,2)^2}=|8.\ 3.\ 0,2|=4,8`
$\\$
`c)` `(\sqrt{20}+\sqrt{5}-\sqrt{45}).\sqrt{5}`
`=(\sqrt{2^2 . 5}+\sqrt{5}-\sqrt{3^2 . 5}).\sqrt{5}`
`=(2\sqrt{5}+\sqrt{5}-3\sqrt{5}).\sqrt{5}`
`=(2+1-3).\sqrt{5}.\sqrt{5}=0.5=0`
$\\$
`d)` `(\sqrt{5}-1)(\sqrt{5}+2)`
`=\sqrt{5}.\sqrt{5}+2\sqrt{5}-\sqrt{5}-2`
`=5-2+(2-1).\sqrt{5}=3+\sqrt{5}`
$\\$
`e)` `(\sqrt{5}+\sqrt{3})^2`
`=(\sqrt{5})^2+2.\sqrt{5}.\sqrt{3}+(\sqrt{3})^2`
`=5+2\sqrt{15}+3=8+2\sqrt{15}`
$\\$
`f)` `(\sqrt{5}-\sqrt{3})^2`
`=(\sqrt{5})^2-2.\sqrt{5}.\sqrt{3}+(\sqrt{3})^2`
`=5-2\sqrt{15}+3=8-2\sqrt{15}`
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`g)` `(\sqrt{5}-\sqrt{3}).(\sqrt{5}+\sqrt{3})`
`=(\sqrt{5})^2-(\sqrt{3})^2`
`=5-3=2`
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Bài 2.
`\qquad 2\sqrt{4+\sqrt{6-2\sqrt{5}}} .(\sqrt{10}-\sqrt{2})`
`=2\sqrt{4+\sqrt{5-2.\sqrt{5}.1+1^2}}. (\sqrt{10}-\sqrt{2})`
`=2\sqrt{4+\sqrt{(\sqrt{5}-1)^2}} .(\sqrt{10}-\sqrt{2})`
`=2\sqrt{4+\sqrt{5}-1} .\sqrt{2}(\sqrt{5}-1)`
`=2\sqrt{3+\sqrt{5}}.\sqrt{2}.(\sqrt{5}-1)`
`=2\sqrt{6+2\sqrt{5}}.(\sqrt{5}-1)`
`=2\sqrt{5+2.\sqrt{5}.1+1^2}(\sqrt{5}-1)`
`=2\sqrt{(\sqrt{5}+1)^2}.(\sqrt{5}-1)`
`=2(\sqrt{5}+1)(\sqrt{5}-1)`
`=2.((\sqrt{5})^2-1^2)=8`
