`\qquad \sqrt{15}-\sqrt{3}`
`=\sqrt{3}.\sqrt{5}-\sqrt{3}`
`=\sqrt{3}(\sqrt{5}-1)`
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`\qquad a\sqrt{b}-b\sqrt{a}` ĐK: `a>=0;b>=0`
`=\sqrt{a}.\sqrt{ab}-\sqrt{b}.\sqrt{ab}`
`=\sqrt{ab}(\sqrt{a}-\sqrt{b})`
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`\qquad a+2\sqrt{a}+1` ĐK: `a>=0`
`=(\sqrt{a}+1)^2`