Tính tổng sau bằng cách hợp lí nhất a) -7/31 + 24/19 + 1/15 + 7/31

* Hướng dẫn giải

a) 

$\frac{-7}{31}+\frac{24}{19}+\frac{1}{15}+\frac{7}{31}+\frac{-5}{19}$$=\left(\frac{-7}{31}+\frac{7}{31}\right)+\left(\frac{24}{19}+\frac{-5}{19}\right)+\frac{1}{15}$$=1+\frac{1}{15}=\frac{16}{15}$

b) 

$\begin{array}{l}\left(\frac{15}{16}+\frac{13}{33}+\frac{1}{11}\right)+\left(\frac{7}{33}+\frac{-15}{16}+\frac{20}{66}\right)\\ =\frac{15}{16}+\frac{13}{33}+\frac{1}{11}+\frac{7}{33}+\frac{-15}{16}+\frac{20}{66}\\ =\left(\frac{15}{16}+\frac{-15}{16}\right)+\left(\frac{13}{33}+\frac{7}{33}\right)+\left(\frac{1}{11}+\frac{20}{66}\right)\\ =\frac{20}{33}+\frac{13}{33}=1\end{array}$