Tính hợp lý a) 8/19 + 4/21 + 2/5 + 17/21 + -27/19 b) -5/18 + 7/4 - -2/3 +5/6

* Hướng dẫn giải

a) 

$\begin{array}{l}\frac{8}{19}+\frac{4}{21}+\frac{2}{5}+\frac{17}{21}+\frac{-27}{19}\\ =\left(\frac{8}{19}+\frac{-27}{19}\right)+\left(\frac{4}{21}+\frac{17}{21}\right)+\frac{2}{5}\\ =-1+1+\frac{2}{5}=\frac{2}{5}\end{array}$

b)

$\begin{array}{l}\frac{-5}{18}+\frac{7}{4}-\frac{-2}{3}+\frac{5}{6}\\ =\frac{-10}{36}+\frac{63}{36}+\frac{24}{36}+\frac{30}{36}\\ =\frac{107}{36}\end{array}$