Tính tổng sau bằng cách hợp lí nhất a) (21/31 + -16/7) + (44/53 + 10/31)

* Hướng dẫn giải

a) 

$\begin{array}{l}\left(\frac{21}{31}+\frac{-16}{7}\right)+\left(\frac{44}{53}+\frac{10}{31}\right)+\frac{9}{53}\\ =\frac{21}{31}+\frac{-16}{7}+\frac{44}{53}+\frac{10}{31}+\frac{9}{53}\\ =\left(\frac{21}{31}+\frac{10}{31}\right)+\left(\frac{44}{53}+\frac{9}{53}\right)+\frac{-16}{7}\\ =1+1+\frac{-16}{7}=\frac{-2}{7}\end{array}$

b) 

$\begin{array}{l}\frac{3}{17}+\frac{-5}{13}+\frac{-18}{35}+\frac{14}{17}+\frac{17}{-35}+\frac{-8}{13}\\ =\left(\frac{3}{17}+\frac{14}{17}\right)+\left(\frac{-5}{13}+\frac{-8}{13}\right)+\left(\frac{-18}{35}+\frac{-17}{35}\right)\\ =1+\left(-1\right)+\left(-1\right)=-1\end{array}$