`c)x-7\sqrtx-8=0(x>=0)`
Đặt `t=\sqrtx(t>=0)`
`pt<=>t^2-7t-8=0`
`<=>t^2-8t+t-8=0`
`<=>t(t-8)+t-8=0`
`<=>(t-8)(t+1)=0`
Mà `t>=0=>t+1>=1>0`
`<=>t-8=0`
`<=>t=8`
`<=>\sqrtx=8`
`<=>x=64(tmđk)`
Vậy `S={64}.`
`a)\sqrt{(\sqrt{11}-4)^2}+\sqrt{11}`
`=|\sqrt{11}-4|+\sqrt{11}`
`=4-\sqrt{11}+\sqrt{11}(do \ 4=\sqrt{16}>\sqrt{11})`
`=4`
`b)\sqrt{(\sqrt3-6)^2}-\sqrt{7-4\sqrt3}`
`=|\sqrt3-6|-\sqrt{4-2.2.\sqrt3+3}`
`=6-\sqrt3-\sqrt{(2-\sqrt3)^2}`
`=6-\sqrt3-2+\sqrt3`
`=4`
