1. Quy đồng mẫu số:
$\dfrac{1}{5}$, $\dfrac{3}{10}$ và $\dfrac{7}{30}$
Mẫu số chung: $30$
$\dfrac{1}{5}=\dfrac{1\times6}{5\times6}=\dfrac{6}{30}$
$\dfrac{3}{10}=\dfrac{3\times3}{10\times3}=\dfrac{9}{30}$
$\dfrac{1}{3}$, $\dfrac{2}{5}$ và $\dfrac{3}{4}$
Mẫu số chung: $60$
$\dfrac{1}{3}=\dfrac{1\times20}{3\times20}=\dfrac{20}{60}$
$\dfrac{2}{5}=\dfrac{2\times12}{5\times12}=\dfrac{24}{60}$
$\dfrac{3}{4}=\dfrac{3\times15}{4\times15}=\dfrac{45}{60}$
2. Quy đồng tử số:
$\dfrac{6}{11}$ và $\dfrac{7}{2}$
Tử số chung: $42$
$\dfrac{6}{11}=\dfrac{6\times7}{11\times7}=\dfrac{42}{77}$
$\dfrac{7}{2}=\dfrac{7\times6}{2\times6}=\dfrac{42}{12}$
$\dfrac{1}{3}$, $\dfrac{2}{5}$ và $\dfrac{3}{4}$
Tử số chung: $6$
$\dfrac{1}{3}=\dfrac{1\times6}{3\times6}=\dfrac{6}{18}$
$\dfrac{2}{5}=\dfrac{2\times3}{5\times3}=\dfrac{6}{15}$
$\dfrac{3}{4}=\dfrac{3\times2}{4\times2}=\dfrac{6}{8}$
3. Tính
$\dfrac{14\times9}{21\times3}=\dfrac{2\times7\times3\times3}{7\times3\times3}=\dfrac{2}{1}=2$
$\dfrac{15\times36\times8}{45\times18\times24}=\dfrac{15\times3\times6\times2\times8}{15\times3\times3\times6\times8\times3}=\dfrac{2}{3\times3}=\dfrac{2}{9}$
