Kết luận nào sau đây là đúng

- Hướng dẫn giải

$$\begin{gathered} B=\frac{\text{x}}{{\text{x}}^{\text{3}}+\text{1}}+\frac{\text{1}-\text{x}}{{\text{x}}^{\text{2}}-\text{x}+\text{1}}+\frac{\text{1}}{\text{x}+\text{1}} \\ =\frac{\text{x}}{\text{(x}+{\text{1)(x}}^{\text{2}}-\text{x}+\text{1)}}+\frac{\text{(1}-\text{x)(x}+\text{1)}}{\text{(x}+{\text{1)(x}}^{\text{2}}-\text{x}+\text{1)}}+\frac{{\text{1.(x}}^{\text{2}}-\text{x}+\text{1)}}{\text{(x}+{\text{1)(x}}^{\text{2}}-\text{x}+\text{1)}} \\ =\frac{\text{x}+\text{1}-{\text{x}}^{\text{2}}+{\text{x}}^{\text{2}}-\text{x}+\text{1}}{\text{(x}+{\text{1)(x}}^{\text{2}}-\text{x}+\text{1)}}=\frac{\text{2}}{{\text{x}}^{\text{3}}+\text{1}} \end{gathered}$$