Bạn nào làm xong mình cho 5 sao

 ĐK `: x >= 0 ; x ne 4`
a) `Q = ((1/(sqrtx + 2) - 2/(x + 4sqrtx + 4)) : (2/(x - 4) - 1/(sqrtx - 2))`
`= [ (sqrtx + 2)/(sqrtx + 2)^2 - 2/(sqrtx + 2)^2] : [2/((sqrtx - 2)(sqrtx + 2)) - (sqrtx + 2)/((sqrtx - 2)(sqrtx + 2))]`
`= sqrtx/(sqrtx + 2)^2 : (-sqrtx)/((sqrtx + 2)(sqrtx - 2))`
`= sqrtx/(sqrtx + 2)^2 . ((sqrtx - 2)(sqrtx + 2))/(-sqrtx)`
`= (2 - sqrtx)/(sqrtx + 2)`
b) `Q = 2/3`
`<=> (2 - sqrtx)/(sqrtx + 2) = 2/3`
`<=> 3(2 - sqrtx) = 2(sqrtx + 2)`
`<=> 6 - 3sqrtx = 2sqrtx + 4`
`<=> 5sqrtx = 2`
`<=> sqrtx = 2/5`
`<=> x = 4/(25) (tm)`
Vậy `x = 4/(25)` thì `Q = 2/3`
c) `Q >= 1/2`
`<=> (2 - sqrtx)/(sqrtx + 2) >= 1/2`
`<=> (2 - sqrtx)/(sqrtx + 2) - 1/2 >= 0`
`<=> (4 - 2sqrtx)/(2(sqrtx + 2)) - (sqrtx + 2)/(2(sqrtx + 2)) >= 0`
`<=> (4 - 2sqrtx - sqrtx - 2)/(2(sqrtx + 2)) >= 0`
`<=> (2- 3sqrtx)/(2(sqrtx + 2)) >= 0`
`<=> 2 - 3sqrtx >= 0` (Vì `: sqrtx >= 0 => sqrtx + 2 > 0)`
`<=> 3sqrtx <= 2`
`<=> sqrtx <= 2/3`
`<=> x <= 4/9`
Kết hợp ĐK `: 0 <= x <= 4/9`
Vậy `0 <= x <= 4/9` thì `Q >= 1/2`