Tính nhanh: B = -4/13 + 3/5 + 1/3 - 9/13 + 2/5

* Hướng dẫn giải

$\begin{array}{l}B=\frac{-4}{13}+\frac{3}{5}+\frac{1}{3}-\frac{9}{13}+\frac{2}{5}=\left(\frac{-4}{13}+\frac{-9}{13}\right)+\left(\frac{3}{5}+\frac{2}{5}\right)+\frac{1}{3}\\ =(-1)+1+\frac{1}{3}=0+\frac{1}{3}=\frac{1}{3}\end{array}$