ĐK : `x \ne 1 , x \ge 0`
`2\sqrt{2-1} = \sqrt{x}/(\sqrt{x}-1)`
`⇔ 2\sqrt{2-1}(\sqrt{x} - 1) = \sqrt{x}/(\sqrt{x}-1) (\sqrt{x}-1)`
`⇔ 2(\sqrt{x}-1) = \sqrt{x}`
`⇔ 2\sqrt{x}-2=\sqrt{x}`
`⇔ 2\sqrt{x} = \sqrt{x} + 2`
`⇔ \sqrt{x} = 2`
`⇔ x = 4(TM)`
Vậy `S ={4}`