Đáp án:
`\sqrt{x - 1} + \sqrt{x + 2} = 3` $\text{(x ≥ 1)}$
`⇔ \sqrt{x - 1} = 3 - \sqrt{x + 2}`
`⇔ x - 1 = 9 - 6\sqrt{x + 2} + x + 2`
`⇔ 6\sqrt{x + 2} = 9 + 2 + 1 + x - x`
`⇔ 6\sqrt{x + 2} = 12`
`⇔ \sqrt{x + 2} = 12 : 6`
`⇔ \sqrt{x + 2} = 2`
`⇔ x + 2 = 4`
`⇔ x = 4 - 2 = 2`
`S = {2}`