`#tnvt`
`\sqrt{1-3x}=4(x<=1/3)`
`=>1-3x=4^2`
`<=>1-3x=16`
`<=>3x=1-16`
`<=>3x=-15`
`<=>x=-5(\text{Thỏa mãn})`
Vậy `S={5}`
$\\$
`\sqrt{x-4\sqrt{x}+4}=5(x>=0)`
`<=>\sqrt{\sqrt{x}^2-2.\sqrt{x}.2+2^2}=5`
`<=>\sqrt{(\sqrt{x}-2)^2}=5`
`<=>|\sqrt{x}-2|=5`
`<=>[(\sqrt{x}-2=5),(\sqrt{x}-2=-5):}`
`<=>[(\sqrt{x}=7),(\sqrt{x}=-3(\text{Loại})):}`
`<=>x=49(\text{Thỏa mãn})`
Vậy `S={49}`
