`x-2\sqrt{x-1}=16` `ĐK: x >= 1`
`<=>x-1-2\sqrt{x-1}-15=0`
Đặt `\sqrt{x-1}=t (t >= 0)` khi đó ta có ptr:
`t^2-2t-15=0`
`<=>t^2-5t+3t-15=0`
`<=>(t-5)(t+3)=0`
`<=>` $\left[\begin{matrix} t=5 (t/m)\\ t=-3(ko t/m)\end{matrix}\right.$
`@t=5=>\sqrt{x-1}=5<=>x-1=25<=>x=26` (t/m)
Vậy `S={26}`