Tính nhanh; A=1/3-3/5+5/7-7/9+9/11

* Hướng dẫn giải

$$\begin{gathered} A=\left(\frac{1}{3}-\frac{1}{3}\right)+\left(\frac{3}{5}-\frac{3}{5}\right)+\left(\frac{5}{7}-\frac{5}{7}\right)+\left(\frac{7}{9}-\frac{7}{9}\right)+\left(\frac{9}{11}-\frac{9}{11}\right)+\left(\frac{11}{13}-\frac{11}{13}\right)+\frac{13}{15} \\ =>A=\frac{13}{15}. \end{gathered}$$