Tính nhanh a) 5/13 + -5/7 + -20/41 + 8/13 + -21/41 b) 1/28 + -1/14

* Hướng dẫn giải

a) $\frac{5}{13}+\frac{-5}{7}+\frac{-20}{41}+\frac{8}{13}+\frac{-21}{41}$$=\left(\frac{5}{13}+\frac{8}{13}\right)+\left(\frac{-20}{41}+\frac{-21}{41}\right)+\frac{-5}{7}=\frac{-5}{7}$

b) $\frac{1}{28}+\frac{-1}{14}+\frac{3}{28}+\frac{-1}{7}+\frac{3}{14}$$=\left(\frac{1}{28}+\frac{3}{28}\right)+\left(\frac{-1}{14}+\frac{3}{14}\right)+\frac{-1}{7}=\frac{1}{7}$

c) $\frac{1}{2}+\frac{-2}{3}+\frac{3}{4}+\frac{-4}{5}+\frac{5}{6}+\frac{-5}{6}+\frac{4}{5}+\frac{-3}{4}+\frac{2}{3}+\frac{-1}{2}$$=\left(\frac{1}{2}+\frac{-1}{2}\right)+\left(\frac{3}{4}+\frac{-3}{4}\right)+\left(\frac{-4}{5}+\frac{4}{5}\right)+\left(\frac{5}{6}+\frac{-5}{6}\right)=0$