Thực hiện phép tính a/(a+1)-a/(a-1)-(2a^2)/(1-a^2) ta được kết quả

* Hướng dẫn giải

Ta có $\frac{\mathrm{a}}{\mathrm{a}+1}-\frac{\mathrm{a}}{\mathrm{a}-1}-\frac{2{\mathrm{a}}^2}{1-{\mathrm{a}}^2}$

$= \frac{\mathrm{a}}{\mathrm{a}+1}-\frac{\mathrm{a}}{\mathrm{a}-1}+\frac{2{\mathrm{a}}^2}{{\mathrm{a}}^2-1}=\frac{\mathrm{a}}{\mathrm{a}+1}-\frac{\mathrm{a}}{\mathrm{a}-1}+\frac{2{\mathrm{a}}^2}{(\mathrm{a}-1\left)\right(\mathrm{a}+1)}$

= $\frac{\mathrm{a}(\mathrm{a}-1)}{(\mathrm{a}+1\left)\right(\mathrm{a}-1)}-\frac{\mathrm{a}(\mathrm{a}+1)}{(\mathrm{a}+1\left)\right(\mathrm{a}-1)}+\frac{2{\mathrm{a}}^2}{(\mathrm{a}+1\left)\right(\mathrm{a}-1)}$

= $\frac{{\mathrm{a}}^2-\mathrm{a}-{\mathrm{a}}^2-\mathrm{a}+2{\mathrm{a}}^2}{(\mathrm{a}+1\left)\right(\mathrm{a}-1)}=\frac{2{\mathrm{a}}^2-2\mathrm{a}}{(\mathrm{a}+1\left)\right(\mathrm{a}-1)}$

= $\frac{2\mathrm{a}(\mathrm{a}-1)}{(\mathrm{a}+1\left)\right(\mathrm{a}-1)}=\frac{2\mathrm{a}}{\mathrm{a}+1}$

Đáp án cần chọn là: C