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Giải thích các bước giải:

Bài `3.`
`1)`
`(sqrt{3}+1)(sqrt{3}-1)=(sqrt{3})^2-1^2=3-1=2`
`3)`
`(2+sqrt{5})(2-sqrt{5})=2^2-(sqrt{5})^2=4-5=-1`
`5)`
`(sqrt{5}+2sqrt{2})(sqrt{5}-2sqrt{2})=(sqrt{5})^2-(2sqrt{2})^2=5-4.2=5-8=-3`
`7)`
`(5+2sqrt{6})(5-2sqrt{6})=5^2-(2sqrt{6})^2=25-24=1`
`9)`
`(20sqrt{12}-15sqrt{27}):5sqrt{3}`
`=(40sqrt{3}-45sqrt{3}):5sqrt{3}`
`=-5sqrt{3}:5sqrt{3}`
`=-1`
`11)`
`(2sqrt{18}-3sqrt{32}+6sqrt{2}):sqrt{2}`
`=(6sqrt{2}-12sqrt{2}+6sqrt{2}):sqrt{2}`
`=(6-12+6)sqrt{2}:sqrt{12}`
`=0`
`13)`
`(sqrt{27}-3sqrt{2}+2sqrt{6}):3sqrt{3}`
`=(3sqrt{3}-3sqrt{2}+2sqrt{6}):3sqrt{3}`
`=3sqrt{3}:3sqrt{3}-3sqrt{2}:3sqrt{3}+2sqrt{6}:3sqrt{3}`
`=1-sqrt{2}/sqrt{3}+2/3 . sqrt{2}`
`=sqrt{3}/sqrt{3}-sqrt{2}/sqrt{3}+(2sqrt{2})/3`
`=(3-sqrt{6}+2sqrt{2})/3`
`15)`
`(sqrt{18}-sqrt{8}:sqrt{2}`
`=sqrt{18}:sqrt{2}-sqrt{8}:sqrt{2}`
`=sqrt{9}-sqrt{4}`
`=3-2`
`=1`