Tính nhanh a) -5/7 + 11/13 + 5/7 b) -5/21 + -2/21 + 8/24

* Hướng dẫn giải

a) $\frac{-5}{7}+\frac{11}{13}+\frac{5}{7}=\left(\frac{-5}{7}+\frac{5}{7}\right)+\frac{11}{13}=\frac{11}{13}$

b) $\frac{-5}{21}+\frac{-2}{21}+\frac{8}{24}=(\frac{-5}{21}+\frac{-2}{21})+\frac{8}{24}=\frac{-7}{21}+\frac{8}{24}=0$