Đáp án:
`a)`
`(5\sqrt{2}-2\sqrt{5}).\sqrt{5}-\sqrt{250}`
`=5\sqrt{2}.\sqrt{5}-2\sqrt{5}.\sqrt{5}-\sqrt{25.10}`
`=5\sqrt{10}-10-5\sqrt{10}`
`=-10`
`b)`
`(2-\sqrt{2})/(1-\sqrt{2})`
`=-(\sqrt{2}-2)/(1-\sqrt{2})`
`=-(\sqrt{2}.(1-\sqrt{2}))/(1-\sqrt{2})`
`=-\sqrt{2}`
`c)`
`\sqrt{11-2\sqrt{30}}-\sqrt{11+2\sqrt{30}}`
`=\sqrt{6-2\sqrt{6.5}+5}-\sqrt{6+2\sqrt{6.5}+5}`
`=\sqrt{(\sqrt{6})^2-2.\sqrt{6}.\sqrt{5}+(\sqrt{5})^2}-\sqrt{(\sqrt{6})^2+2\sqrt{6}.\sqrt{5}+(\sqrt{5})^2}`
`=\sqrt{(\sqrt{6}-\sqrt{5})^2}-\sqrt{(\sqrt{6}+\sqrt{5})^2}`
`=|\sqrt{6}-\sqrt{5}|-|\sqrt{6}+\sqrt{5}|`
`=\sqrt{6}-\sqrt{5}-\sqrt{6}-\sqrt{5}(Vì:\sqrt{6}>\sqrt{5})`
`=-2\sqrt{5}`
