Đáp án + Giải thích các bước giải:
$\rm Bài \ 1 : \\ a) \ MSC : 84 \\ \dfrac{-1}{7}=\dfrac{-1.12}{7.12}=\dfrac{-12}{84} \\ \dfrac{-5}{21}=\dfrac{-5.4}{21.4}=\dfrac{-20}{84} \\ \dfrac{-2}{3}=\dfrac{-2.28}{3.28}=\dfrac{-56}{84} \\ \dfrac{-2}{14}=\dfrac{-2.6}{14.6}=\dfrac{-12}{84} \\ Vì -12>-20>-56 \\ \to \dfrac{-1}{7}=\dfrac{-2}{14} > \dfrac{-5}{21} > \dfrac{-2}{3} \\ b) \ (\dfrac{16}{9}x-10).\dfrac{5}{2}=55 \\ \to \dfrac{16}{9}x-10=55:\dfrac{5}{2}=22 \\\to \dfrac{16}{9}x=22+10=32 \\ \to x=32:\dfrac{16}{9}=18 \\ Vậy \ x=18 \\ Bài \ 2 : \\ a) \ A=(\dfrac{5}{9}-\dfrac{1}{4})+(\dfrac{4}{7}-\dfrac{1}{14}).\dfrac{50}{36} \\ =\dfrac{11}{36}+\dfrac{1}{2}.\dfrac{50}{36}\\=\dfrac{11}{36}+\dfrac{25}{36}=1 \\ b) \ B=\dfrac{ (2 \dfrac{1}{5}-\dfrac{3}{5}):\dfrac{16}{18} }{\dfrac{9}{10}}=\dfrac{\dfrac{8}{5} .\dfrac{9}{8} }{\dfrac{9}{10}}=\dfrac{\dfrac{9}{5}}{\dfrac{9}{10}}=2$
