Ta có :
$a )$ $\dfrac{5}{45}$ = $\dfrac{5 : 5}{45 : 5}$ = $\dfrac{1}{9}$
$\dfrac{39}{65}$ = $\dfrac{39}{65}$ ( đã ở dạng tối giản )
$\dfrac{112}{140}$ = $\dfrac{112 : 28}{140:28}$ = $\dfrac{4}{5}$
b) $\dfrac{18}{24}$ = $\dfrac{18 : 6}{24 : 6}$ = $\dfrac{3}{4}$
$\dfrac{15}{20}$ = $\dfrac{15 : 5}{20 : 5}$ = $\dfrac{3}{4}$
$\dfrac{54}{96}$ = $\dfrac{54 : 6}{96 : 6}$ = $\dfrac{9}{16}$