` c) ` ` \sqrt{9x + 18} + 15\sqrt{\frac{x+2}{9}} = 16 `
` <=> \sqrt{9(x + 2)} + 15.\frac{\sqrt{x+2}}{\sqrt{9}} = 16 `
` <=> \sqrt{9(x + 2)} + 15.\frac{\sqrt{x+2}}{3} = 16 `
` <=> 3\sqrt{9(x + 2)} + 5\sqrt{x + 2} = 16 `
` <=> 8\sqrt{x + 2} = 16 `
` <=> \sqrt{x + 2} = 2 `
` <=> \sqrt{x + 2} = \sqrt{4} `
` <=> x + 2 = 4 `
` <=> x = 2 `
Vậy ` S = {2} `
