`#tnvt`
`a)2\sqrt{x-5}=x-5(x>=5)`
`<=>(x-5)-2\sqrt{x-5}=0`
`<=>\sqrt{x-5}(\sqrt{x-5}-2)=0`
`<=>[(x-5=0),(\sqrt{x-5}=2):}`
`<=>[(x=5(tm)),(x-5=4):}`
`<=>[(x=5(tm)),(x=9(tm)):}`
Vậy `S={5;9}`
`b)\sqrt{x+6\sqrt{x}+9}-6=\sqrt{9-6\sqrt{x}+x}(x>=0)`
`<=>\sqrt{(\sqrt{x}+3)^2}-6=\sqrt{(3-\sqrt{x})^2}`
`<=>|\sqrt{x}+3|-6=|3-\sqrt{x}|`
`<=>\sqrt{x}+3-6=|3-\sqrt{x}|`
`<=>|3-\sqrt{x}|=\sqrt{x}-3(x>=9)`
`<=>[(3-\sqrt{x}=\sqrt{x}-3),(\sqrt{x}-3=\sqrt{x}-3):}`
`<=>[(2\sqrt{x}=6),(0=0(\text{Luôn đúng})):}`
`<=>\sqrt{x}=3`
`<=>x=9(tm)`
Vậy `S={9}`
