Lời giải
Câu `1:`
`A=\sqrt{5}(\sqrt{20}-\sqrt{5})+1`
`A=\sqrt{5}(\sqrt{4.5}-\sqrt{5.1})+1`
`A=\sqrt{5}.\sqrt{5}.(\sqrt{4}-\sqrt{1})+1`
`A=5.(2-1)+1`
`A=5.1+1`
`A=6.`
Câu `2:`
`2\sqrt{75}+3\sqrt{48}-4\sqrt{27}`
`=2\sqrt{25.3}+3\sqrt{16.3}-4\sqrt{9.3}`
`=2.\sqrt{25}. \sqrt{3}+3.\sqrt{16}. \sqrt{3}-4.\sqrt{9}. \sqrt{3}`
`=2.5.\sqrt{3}+3. 4. \sqrt{3} - 4. 3. \sqrt{3}`
`=10\sqrt{3}+12\sqrt{3}-12\sqrt{3}`
`=\sqrt{3}(10+12-12)`
`=10\sqrt{3}.`
