`#tnvt`
`\sqrt{25-x^2}=x-1`
Đk: `{(25-x^2>=0),(x-1>=0):}`
`<=>{(25>=x^2),(x>=1):}`
`<=>{(-5<=x<=5),(x>=1):}`
`->ĐKXĐ: 1<=x<=5`
`\sqrt{25-x^2}=x-1(1<=x<=5)`
`<=>25-x^2=(x-1)^2`
`<=>25-x^2=x^2-2x+1`
`<=>2x^2-2x-24=0`
`<=>x^2-x-12=0`
`<=>x^2+3x-4x-12=0`
`<=>(x+3)(x-4)=0`
`<=>[(x=-3(ktm)),(x=4(tm)):}`
Vậy `S={4}`