`(2-(a-3\sqrt{a})/(\sqrt{a}-3))(2-(5\sqrt{a}-\sqrt{ab})/(\sqrt{b}-5))` `(đk: a, b ≥ 0; a ne 9; b ne 25)`
`=(2-(\sqrt{a}(\sqrt{a}-3))/(\sqrt{a}-3))(2-(\sqrt{a}(5-\sqrt{b}))/(\sqrt{b}-5))`
`=(2-(\sqrt{a}(\sqrt{a}-3))/(\sqrt{a}-3))(2+(\sqrt{a}(5-\sqrt{b}))/(5-\sqrt{b}))`
`=(2-\sqrt{a})(2+\sqrt{a})`
`=4-a`