giúp em với ạ em cảm ơn

`B = (2 sqrtx)/(sqrtx + 3) + (sqrtx)/(sqrtx - 3)` `(x >= 0; x ne 9)`

`= (2 sqrtx (sqrtx - 3))/((sqrtx + 3)(sqrtx - 3)) + (sqrtx(sqrtx + 3))/((sqrtx - 3)(sqrtx + 3))`

`= (2x - 6 sqrtx + x + 3 sqrtx)/((sqrtx - 3)(sqrtx + 3))`

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

`= (sqrtx(sqrtx - 3))/((sqrtx - 3)(sqrtx + 3))`

`= (sqrtx)/(sqrtx + 3)`

Vậy `B = (sqrtx)/(sqrtx + 3)` với `x >= 0; x ne 9`

`----------`

`C = (2 sqrtx)/(sqrtx + 3) + (sqrtx)/(sqrtx - 3) - (3x + 9)/(x - 9)` `(x >= 0; x ne 9)`

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

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

`= (-3 sqrtx - 9)/((sqrtx - 3)(sqrtx + 3))`

`= (-3 (sqrtx + 3))/((sqrtx - 3)(sqrtx + 3))`

`= (-3)/(sqrtx-3)`

Vậy `C = (-3)/(sqrtx-3)` với `x >= 0; x ne 9`