a) Ta có: 12= $2^{2}$ . 3 ; 15= 3.5

B C N N ( 12 , 15 )= $2^{2}$ .3 .5= 60 nên chọn mẫu chung là 60.

$\dfrac{9}{12}$= $\dfrac{9.5}{12.5}$= $\dfrac{45}{60}$

$\dfrac{7}{15}$= $\dfrac{7.4}{15.4}$= $\dfrac{28}{60}$

b) Ta có: 10= 2.5 ; 4= $2^{2}$ ; 14= 2.7

B C N N ( 10 , 4 , 14 )= 2 2 .5 .7

140 nên chọn mẫu chung là 140.

$\dfrac{7}{10}$= $\dfrac{7.14}{10.14}$= $\dfrac{98}{140}$

$\dfrac{3}{4}$= $\dfrac{3.35}{4.35}$= $\dfrac{105}{140}$

$\dfrac{9}{14}$= $\dfrac{9.10}{14.10}$= $\dfrac{90}{140}$