Bài 1:

a) \(\dfrac{5}{18}\) và \(\dfrac{4}{3}\)   

Mẫu số chung:  \(18\)

\(\dfrac{4}{3}=\dfrac{4\times6}{3\times6}=\dfrac{24}{18}\)

b) \(\dfrac{7}{5}\) và \(\dfrac{8}{11}\)

Mẫu số chung:  \(55\)

\(\dfrac{7}{5}=\dfrac{7\times11}{5\times11}=\dfrac{77}{55}\)   ;    \(\dfrac{8}{11}=\dfrac{8\times5}{11\times5}=\dfrac{40}{55}\)

c) \(\dfrac{7}{12}\) và \(\dfrac{5}{6}\)

Mẫu số chung:  \(12\)

\(\dfrac{5}{6}=\dfrac{5\times2}{6\times2}=\dfrac{10}{12}\)

Bài 2:

a)  \(\dfrac{5}{10}\) và \(\dfrac{25}{75}\)

\(\dfrac{5}{10}=\dfrac{5:5}{10:5}=\dfrac{1}{2}\) ;   \(\dfrac{25}{75}=\dfrac{25:25}{75:25}=\dfrac{1}{3}\)

Mẫu số chung:  6

\(\dfrac{1}{2}=\dfrac{1\times3}{2\times3}=\dfrac{2}{6}\) ;  \(\dfrac{1}{3}=\dfrac{2\times2}{3\times2}=\dfrac{4}{6}\)

b)  \(\dfrac{42}{56}\) và \(\dfrac{18}{48}\)

\(\dfrac{42}{56}=\dfrac{42:14}{56:14}=\dfrac{3}{4}\) ;  \(\dfrac{18}{48}=\dfrac{18:6}{48:6}=\dfrac{3}{8}\)

Mẫu số chung:  8

\(\dfrac{3}{4}=\dfrac{3\times2}{4\times2}=\dfrac{6}{8}\)

Bài 3:

a) \(\dfrac{12\times5}{4\times9\times5}\) \(=\dfrac{3\times4\times5}{4\times3\times3\times5}=\dfrac{1}{3}\)

b)  \(\dfrac{32\times50}{48\times75}\) \(=\dfrac{4\times8\times25\times2}{2\times3\times8\times25\times3}=\dfrac{4}{9}\)

c)  \(\dfrac{63\times81}{21\times27}\) \(=\dfrac{7\times9\times3\times3\times9}{3\times7\times3\times9}=9\)

d) \(\dfrac{11\times10}{4\times55}\) \(=\dfrac{11\times5\times2}{2\times2\times11\times5}=\dfrac{1}{2}\)