Bài 7:
\(a,18+\dfrac{9}{3}=\dfrac{54}{3}+\dfrac{9}{3}=\dfrac{63}{3}=21\\ b,\dfrac{4}{9}-\dfrac{5}{18}=\dfrac{8}{18}-\dfrac{5}{18}=\dfrac{3}{18}=\dfrac{1}{6}\\ c,\dfrac{3}{5}:\dfrac{5}{7}=\dfrac{3}{5}\times\dfrac{7}{5}=\dfrac{21}{25}\\ d,\dfrac{4}{7}\times\dfrac{2}{7}=\dfrac{8}{49}\)
Bài 8:
\(a,\dfrac{5}{4}\times x=\dfrac{3}{8}+\dfrac{5}{12}\\ \dfrac{5}{4}\times x=\dfrac{9}{24}+\dfrac{10}{24}\\ \dfrac{5}{4}\times x=\dfrac{19}{24}\\ x=\dfrac{19}{24}:\dfrac{5}{4}\\ x=\dfrac{19}{24}\times\dfrac{4}{5}\\ x=\dfrac{19}{30}\)
\(3\times\dfrac{7}{10}+\dfrac{7}{10}\times5+2\times\dfrac{7}{10}\\ =\dfrac{7}{10}\times\left(3+5+2\right)\\ =\dfrac{7}{10}\times10\\ =7\)
