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Đáp án + Giải thích các bước giải:

 `a)`

`A=3\sqrt{45}-3\sqrt{180}+2\sqrt{20}`

`A= 3\sqrt{9.5}-3\sqrt{36.5}+2\sqrt{4.5}`

`A=3\sqrt{3^2 .5}-3\sqrt{6^2 .5}+2\sqrt{2^2 .5}`

`A= 3.3\sqrt{5}-3.6\sqrt{5}+2.2\sqrt{5}`

`A=9\sqrt{5}-18\sqrt{5}+4\sqrt{5}`

`A= -5\sqrt{5}`

Vậy `A=-5\sqrt{5}`.

`b)`

`B= (2/(\sqrt{x}+2)-1/(\sqrt{x}-2)) .(\sqrt{x}+2)/(\sqrt{x}-6)` với `x>=0; x\ne4; x\ne36`

`B= ((2(\sqrt{x}-2))/((\sqrt{x}-2)(\sqrt{x}+2)) -(\sqrt{x}+2)/((\sqrt{x}-2)(\sqrt{x}+2))) .(\sqrt{x}+2)/(\sqrt{x}-6)`

`B= (2(\sqrt{x}-2)-\sqrt{x}-2)/((\sqrt{x}-2)(\sqrt{x}+2)) . (\sqrt{x}+2)/(\sqrt{x}-6)`

`B= (2\sqrt{x}-4 - \sqrt{x}-2)/((\sqrt{x}-2)(\sqrt{x}+2)) .(\sqrt{x}+2)/(\sqrt{x}-6)`

`B= (\sqrt{x}-6)/((\sqrt{x}-2)(\sqrt{x}+2)) .(\sqrt{x}+2)/(\sqrt{x}-6)`

`B= 1/(\sqrt{x}-2)`

Vậy `B=1/(\sqrt{x}-2)`