Bài 4:
\(\left(\dfrac{5}{4}+\dfrac{3}{4}\right)\cdot\dfrac{2}{3}=\dfrac{8}{4}\cdot\dfrac{2}{3}=2\cdot\dfrac{2}{3}=\dfrac{4}{3}\)
\(\dfrac{5}{4}\cdot\dfrac{2}{3}+\dfrac{3}{4}\cdot\dfrac{2}{3}\)
\(=\dfrac{5\cdot2}{4\cdot3}+\dfrac{3\cdot2}{4\cdot3}\)
\(=\dfrac{10}{12}+\dfrac{6}{12}=\dfrac{16}{12}=\dfrac{4}{3}\)
\(\left(\dfrac{3}{2}-\dfrac{3}{8}\right)\cdot\dfrac{2}{7}=\dfrac{3\cdot4-3}{8}\cdot\dfrac{2}{7}\)
\(=\dfrac{3\left(4-1\right)}{8}\cdot\dfrac{2}{7}=\dfrac{3\cdot3}{8}\cdot\dfrac{2}{7}=\dfrac{3\cdot3}{4\cdot7}=\dfrac{9}{28}\)
\(\dfrac{3}{2}\cdot\dfrac{2}{7}-\dfrac{3}{8}\cdot\dfrac{2}{7}\)
\(=\dfrac{3}{7}-\dfrac{3}{28}\)
\(=\dfrac{3\cdot4-3}{28}=\dfrac{9}{28}\)
Bài 3:
\(\dfrac{5}{6}\cdot\dfrac{4}{5}\cdot\dfrac{3}{2}=\dfrac{5}{5}\cdot\dfrac{4}{2}\cdot\dfrac{3}{6}=1\cdot2\cdot\dfrac{1}{2}=1\)
\(\dfrac{9}{4}\cdot\left(\dfrac{5}{2}\cdot\dfrac{2}{5}\right)\)
\(=\dfrac{9}{4}\cdot\dfrac{5\cdot2}{2\cdot5}\)
\(=\dfrac{9}{4}\cdot\dfrac{10}{10}=\dfrac{9}{4}\)
\(\left(\dfrac{3}{7}\cdot\dfrac{5}{6}\right)\cdot\dfrac{5}{2}=\dfrac{3\cdot5}{7\cdot6}\cdot\dfrac{5}{2}=\dfrac{3}{6}\cdot\dfrac{5}{7}\cdot\dfrac{5}{2}\)
\(=\dfrac{1}{2}\cdot\dfrac{25}{14}=\dfrac{25}{28}\)
