Tính các tổng sau một cách hợp lí: 4/7 +4/28+9/21

* Hướng dẫn giải

$\begin{array}{l}a)\frac{4}{7}+\frac{1}{7}+\frac{3}{7}=\frac{8}{7}\\ \\ b)\frac{4}{7}+\frac{-3}{7}+\frac{-1}{2}+\frac{1}{2}=\frac{1}{7}\end{array}$

$\begin{array}{l}c)\left(\frac{-27}{10}+\frac{-3}{10}\right)+\left(\frac{1}{4}+\frac{5}{4}\right)+\frac{13}{2}=-3+\frac{3}{2}+\frac{13}{2}=5\\ \\ d)\left(\frac{8}{5}+\frac{7}{5}\right)+\left(\frac{-11}{4}+\frac{-9}{4}\right)+\frac{-7}{20}=3-5+\frac{-7}{20}=\frac{-47}{20}\\ \\ e)\left(\frac{7}{5}+\frac{-1}{15}\right)+\left(\frac{-11}{3}+\frac{11}{2}+\frac{1}{6}\right)+(-2)=\frac{4}{3}+2+(-2)=\frac{4}{3}\end{array}$