Tính nhanh a) 4/7 + 3/4 + 2/7 + 5/4 +1/7 b) -5/7 + 3/4 + -1/5 + -2/7 +1/4

* Hướng dẫn giải

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

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