`-` ĐKXĐ: `{(x >= 0),(x ne 9):}`

`A = sqrt(x)/(sqrt(x) + 3) + (2sqrt(x))/(sqrt(x) - 3) - (3x + 9)/(x - 9)`

`= (sqrt(x)(sqrt(x) - 3))/((sqrt(x) + 3)(sqrt(x) - 3)) + (2sqrt(x)(sqrt(x) + 3))/((sqrt(x) - 3)(sqrt(x) + 3)) - (3x + 9) / ((sqrt(x) + 3)(sqrt(x) - 3))`

`= (sqrt(x)(sqrt(x) - 3) + 2sqrt(x)(sqrt(x) + 3) - (3x + 9))/((sqrt(x) + 3)(sqrt(x) - 3))`

`= (x - 3sqrt(x) + 2x + 6sqrt(x) - 3x - 9)/((sqrt(x) + 3)(sqrt(x) - 3))`

`= (3sqrt(x) - 9)/((sqrt(x) + 3)(sqrt(x) - 3))`

`= (3(sqrt(x) - 3))/((sqrt(x) + 3)(sqrt(x) - 3))`

`= 3/(sqrt(x) + 3)`

Để `A = 1/3` thì

`3 / (sqrt(x) + 3) = 1/3`

`=> sqrt(x) + 3 = 9`

`=> sqrt(x) = 6`

`=> x = 36` (tmđk)

Vậy `x = 36` thì `A = 1/3`

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