ĐKXĐ `a \ne b ; a ; b >= 0 `

`( a - b )/( \sqrt{a} - \sqrt{b} ) - (\sqrt{a^3} - \sqrt{b^3} )/( a + b + \sqrt{ab} )`

`= ((\sqrt{a})^2-(\sqrt{b})^2)/(\sqrt{a}-\sqrt{b}) - ((\sqrt{a})^3-(\sqrt{b})^3)/(a+b+\sqrt{ab})`

`= ((\sqrt{a} - \sqrt{b} )( \sqrt{a} + \sqrt{b} ))/(\sqrt{a} - \sqrt{b} ) - ((\sqrt{a} - \sqrt{b} )( a + \sqrt{ab} + b ))/( a + b + \sqrt{ab} )`

`= \sqrt{a}  + \sqrt{b} - ( \sqrt{a} - \sqrt{b}  )`

`= \sqrt{a} + \sqrt{b} - \sqrt{a} + \sqrt{b}`

`= 2\sqrt{b}`