Tính hợp lý a) 5/8 + 3/17 + 4/18 + 20/-17 + -2/9 + 21/56 b) -2/11

* Hướng dẫn giải

a) $\frac{5}{8}+\frac{3}{17}+\frac{4}{18}+\frac{20}{-17}+\frac{-2}{9}+\frac{21}{56}$

$\begin{array}{l}=\frac{5}{8}+\frac{3}{17}+\frac{2}{9}+\frac{-20}{17}+\frac{-2}{9}+\frac{3}{8}\\ =\left(\frac{5}{8}+\frac{3}{8}\right)+\left(\frac{3}{17}+\frac{-20}{17}\right)+\left(\frac{2}{9}+\frac{-2}{9}\right)\\ =1+\left(-1\right)+0=0\end{array}$

b) $\frac{-2}{11}+\frac{6}{7}+\frac{1}{2}+\frac{-9}{11}+\frac{1}{7}$

$\begin{array}{l}=\left(\frac{-2}{11}+\frac{-9}{11}\right)+\left(\frac{6}{7}+\frac{1}{7}\right)\\ =-1+1=0\end{array}$