Đáp án:
`a)ĐK:1-x>=0<=>x<=1`
`sqrt{1-x}>2`
`<=>1-x>4`
`<=>x<1-4`
`<=>x<-3`
`b)sqrt4-x<=2`
`<=>2-x<=2`
`<=>x>=0`
`a)sqrt{x^2+4x+5}`
`=sqrt{x^2+4x+4+1}`
`=sqrt{(x+2)^2+1}`
Vì `(x+2)^2>=0`
`=>(x+2)^2+1>=1`
`=>sqrt{(x+2)^2+1}>=1`
Dấu "=" xảy ra khi `x+2=0<=>x=-2`
`b)sqrt{x^2+2x+5}`
`=sqrt{x^2+2x+1+4}`
`=sqrt{(x+1)^2+4}`
Vì `(x+1)^2>=0`
`=>(x+1)^2+4>=4`
`=>sqrt{(x+1)^2+4}>=2`
Dấu "=" xảy ra khi `x+1=0<=>x=-1`