`19.`
`a)\sqrt{0,36a^2}` với `a<0`
`=\sqrt{(0,6a)^2}`
`=|0,6a|`
`=-0,6a`
`b)\sqrt{a^4(3-a)^2}` với `a>=3`
`=\sqrt{(a^2)^2}.\sqrt{(3-a)^2}`
`=a^2.|3-a|`
`=a^2(a-3)`
`c)\sqrt{27.48(1-a)^2}` với `a>1`
`=\sqrt{3.9.16.3.(1-a)^2}`
`=\sqrt{81.16.(1-a)^2}`
`=\sqrt{81}.\sqrt{16}.\sqrt{(1-a)^2}`
`=9.4.|1-a|`
`=36.(a-1)`
`d)1/(a-b).\sqrt{a^4(a-b)^2}` với `a>b`
`=1/(a-b).\sqrt{a^4}.\sqrt{(a-b)^2}`
`=1/(a-b).a^2.|a-b|`
`=1/(a-b).a^2(a-b)`
`=a^2`
`20.`
`a)\sqrt{(2a)/3}.\sqrt{(3a)/8` với `a>=0`
`=\sqrt{(2a.3a)/(3.8)`
`=\sqrt{(6a^2)/(6.4)}`
`=\sqrt{(a^2)/4}`
`=\sqrt{(a/2)^2}`
`=|a/2|`
`=a/2`
`b)\sqrt{13a}.\sqrt{52/a}` với `a>0`
`=\sqrt{(13a.52)/a}`
`=\sqrt{13.13.4}`
`=sqrt{13^2}.\sqrt{2^2}`
`=13.2`
`=26`
`c)\sqrt{5a}.\sqrt{45a}-3a` với `a>=0`
`=\sqrt{5a.45a}-3a`
`=\sqrt{5a.5.9.a}-3a`
`=\sqrt{5^2 . a^2 . 3^2}-3a`
`=\sqrt{a^2}.\sqrt{5^2} . \sqrt{3^2}-3a`
`=|a|.5.3-3a`
`=15a-3a`
`=12a`
`d)(3-a)^2-\sqrt{0,2}.\sqrt{180a^2}`
`=(3-a)^2-\sqrt{0,2.180a^2}`
`=(3-a)^2-\sqrt{0,2.18.10.a^2}`
`=(3-a)^2-\sqrt{2.18.a^2}`
`=(3-a)^2-\sqrt{36a^2}`
`=(3-a)^2-\sqrt{6}.\sqrt{a^2}`
`=(3-a)^2-6.|a|`
`+)` Nếu `a>=0`
`->(3-a)^2-6|a|`
`=9-6a+a^2-6a`
`=9-(6a+6a)+a^2`
`=9-12a+a^2`
`+)` Nếu `a<0`
`->(3-a)^2-6.|a|`
`=9-6a+a^2+6a`
`=9-(6a-6a)+a^2`
`=a^2+9`
