A= 3/x+3+1/x-3+18/x^2-9

`A = \frac{3}{x + 3} + \frac{1}{x - 3} + \frac{18}{x^2 - 9}` ( với `x \ne ± 3` )

`=> A = \frac{3(x - 3)}{(x + 3)(x - 3)} + \frac{x + 3}{(x - 3)(x + 3)} + \frac{18}{(x - 3)(x + 3)}`

`=> A = \frac{3x - 9 + x + 3 + 18}{(x - 3)(x + 3)}`

`=> A = \frac{4x + 12}{(x - 3)(x + 3)}`

`=> A = \frac{4( x + 3)}{(x - 3)( x + 3)}`

`=> A = \frac{4}{x - 3}`