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Đáp án+Giải thích các bước giải:

 `61)`

`\frac{a-2\sqrt{a}}{2-\sqrt{a}}` `(x\ge0;x\ne4)`

`=\frac{\sqrt{a}.(\sqrt{a}-2)}{-(\sqrt{a}-2)}`

`=\frac{-\sqrt{a}}{1}`

`=-\sqrt{a}`

`62)`

`\frac{1-a}{1+\sqrt{a}}` `(a\ge0)`

`=\frac{1^{2}-(\sqrt{a})^{2}}{1+\sqrt{a}}`

`=\frac{(1-\sqrt{a}).(1+\sqrt{a})}{1+\sqrt{a}}`

`=1-\sqrt{a}`

`63)`

`\frac{x^{2}-3}{x+\sqrt{3}}` `(x\ne-\sqrt{3})`

`=\frac{x^{2}-(\sqrt{3})^{2}}{x+\sqrt{3}}`

`=\frac{(x-\sqrt{3}).(x+\sqrt{3})}{x+\sqrt{3}}`

`=x-\sqrt{3}`

`64)`

`\frac{4-a}{\sqrt{a}-2}(a\ge0;a\ne4)`

`=\frac{2^{2}-(\sqrt{a})^{2}}{\sqrt{a}-2}`

`=\frac{(2-\sqrt{a}).(2+\sqrt{a})}{-(2-\sqrt{a})}`

`=\frac{-(2+\sqrt{a})}{1}`

`=-2-\sqrt{a}`

`65)`

`\frac{a-1}{\sqrt{a}-1}(a\ge0;a\ne1)`

`=\frac{(\sqrt{a})^{2}-1^{2}}{\sqrt{a}-1}`

`=\frac{(\sqrt{a}-1).(\sqrt{a}+1)}{\sqrt{a}-1}`

`=\sqrt{a}+1`