Thu gọn biểu thức ta được A= (3x+21)/(x^2-9)+2/(x+3)-3/

* Hướng dẫn giải

Ta có A=$\frac{3\mathrm{x}+21}{{\mathrm{x}}^2-9}+\frac{2}{\mathrm{x}+3}-\frac{3}{\mathrm{x}-3}$

=$\frac{3\mathrm{x}+21}{(\mathrm{x}-3\left)\right(\mathrm{x}+3)}+\frac{2(\mathrm{x}-3)}{(\mathrm{x}-3\left)\right(\mathrm{x}+3)}-\frac{3(\mathrm{x}+3)}{(\mathrm{x}-3\left)\right(\mathrm{x}+3)}$

=$\frac{3\mathrm{x}+21+2\mathrm{x}-6-3\mathrm{x}-9}{9\mathrm{x}-3\left)\right(\mathrm{x}+3)}=\frac{2\mathrm{x}+6}{(\mathrm{x}-3\left)\right(\mathrm{x}+3)}$

=$\frac{2(\mathrm{x}+3)}{(\mathrm{x}-3\left)\right(\mathrm{x}+3)}=\frac{2}{\mathrm{x}-3}$

Đáp án cần chọn là: D