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Đáp án`+`Giải thích các bước giải:

 `A = 1/(x^2 + 6x +9) + 1/(6x -x^2 -9) + x/(x^2- 9)` (ĐK:`x \ne +-3`)

`=> A = 1/(x^2+ 6x + 9) - 1/(x^2 - 6x + 9) + x/(x^2 - 9)`

`=> A = 1/((x + 3)^2) - 1/((x - 3)^2) + x/((x - 3)(x +3))`

`=> A = ((x- 3)^2 - (x + 3)^2 + x(x - 3)(x +3))/((x - 3)^2 (x + 3)^2)`

`=> A = (x^2- 6x + 9 - x^2- 6x - 9 +x(x^2 - 9))/((x - 3)^2 (x+ 3)^2)`

`=> A= (-12x + x^3 - 9x)/((x - 3)^2 (x + 3)^2)`

`=> A = (x^3 - 21x)/((x - 3)^2 (x+ 3)^2)`

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