Bài 2.1:
a) ($\dfrac{-5}{3}$ + $\dfrac{2}{5}$) : $\dfrac{4}{5}$ + ($\dfrac{-1}{3}$ + $\dfrac{8}{5}$) : $\dfrac{4}{5}$
= [$\dfrac{-5}{3}$ + $\dfrac{2}{5}$ + ($\dfrac{-1}{3}$) + $\dfrac{8}{5}$] : $\dfrac{4}{5}$
= [$\dfrac{-5}{3}$ + ($\dfrac{-1}{3}$) + ($\dfrac{2}{5}$ + $\dfrac{8}{5}$)] : $\dfrac{4}{5}$
= [-2 + 2] : $\dfrac{4}{5}$
= 0 : $\dfrac{4}{5}$
= 0
b) $\dfrac{15}{34}$ + $\dfrac{7}{21}$ + $\dfrac{19}{34}$ - $\dfrac{15}{20}$ + $\dfrac{3}{7}$
= ($\dfrac{15}{34}$ + $\dfrac{19}{34}$) + ($\dfrac{7}{21}$ + $\dfrac{3}{7}$) + (- $\dfrac{3}{4}$)
= 1 + $\dfrac{16}{21}$ + (- $\dfrac{3}{4}$)
= $\dfrac{37}{21}$ + (- $\dfrac{3}{4}$)
= $\dfrac{85}{84}$
Bài 2.2:
a) $\dfrac{5}{7}$ - ($\dfrac{4}{3}$ + x) = $\dfrac{3}{7}$
($\dfrac{4}{3}$ + x) = $\dfrac{5}{7}$ - $\dfrac{3}{7}$
$\dfrac{4}{3}$ + x = $\dfrac{2}{7}$
x = $\dfrac{2}{7}$ - $\dfrac{4}{3}$
x = - $\dfrac{22}{21}$
b) $\dfrac{3}{4}$ + $\dfrac{1}{4}$ : x = $\dfrac{2}{5}$
$\dfrac{1}{4}$ : x = $\dfrac{2}{5}$ - $\dfrac{3}{4}$
$\dfrac{1}{4}$ : x = $\dfrac{-7}{20}$
x = $\dfrac{1}{4}$ : $\dfrac{-7}{20}$
x = $\dfrac{-5}{7}$
