Đáp án:
`3)` `P=-5`
`4)` `Q=1`
Giải thích các bước giải:
`3)` `P=\sqrt{x^2-10x+25}-x` `(x\ge 5)`
`=\sqrt{x^2-2.x.5+5^2}-x`
`=\sqrt{(x-5)^2}-x`
`=|x-5|-x`
`=x-5-x` (vì `x\ge 5=>x-5\ge 0`)
`=-5`
Vậy `P=5`
$\\$
`4)` `Q=3x+2-\sqrt{9x^2+6x+1}` `(x\ge 1/3)`
`=3x+2-\sqrt{(3x)^2+2.3x.1+1^2}`
`=3x+2-\sqrt{(3x+1)^2}`
`=3x+2-|3x+1|`
`=3x+2-(3x+1)`
(vì `x\ge 1/3=>3x\ge 1=>3x+1\ge 2>0)`
`=3x+2-3x-1=1`
Vậy `Q=1`
