Tính nhanh a) 1/5 + 2/9 + 4/5 + -11/9 + 3/7 b) -7/31 + 24/17 + 7/31

* Hướng dẫn giải

a) $\frac{1}{5}+\frac{2}{9}+\frac{4}{5}+\frac{-11}{9}+\frac{3}{7}=\left(\frac{1}{5}+\frac{4}{5}\right)+\left(\frac{2}{9}+\frac{-11}{9}\right)+\frac{3}{7}$$=1+(-1)+\frac{3}{7}=\frac{3}{7}$

b) $\frac{-7}{31}+\frac{24}{17}+\frac{7}{31}+\frac{-5}{17}+1=\left(\frac{-7}{31}+\frac{7}{31}\right)+\left(\frac{24}{17}+\frac{-5}{17}\right)+1$$=0+\frac{19}{17}+1=\frac{36}{17}$