342.

a, $\dfrac{4}{5}$ và $\dfrac{1}{2}$

$\dfrac{4}{5}$ = $\dfrac{4 × 2}{5 × 2}$ = $\dfrac{8}{10}$

$\dfrac{1}{2}$ = $\dfrac{1 × 5}{2 × 5}$ = $\dfrac{5}{10}$

b, $\dfrac{5}{8}$ và $\dfrac{3}{5}$ 

$\dfrac{5}{8}$ = $\dfrac{5 × 5}{8 × 5}$ = $\dfrac{25}{40}$

$\dfrac{3}{5}$ = $\dfrac{3 × 8}{5 × 8}$ = $\dfrac{24}{40}$

c, $\dfrac{7}{8}$ và $\dfrac{8}{7}$

$\dfrac{7}{8}$ = $\dfrac{7 × 7}{8 × 7}$ = $\dfrac{49}{56}$

$\dfrac{8}{7}$ = $\dfrac{8 × 8}{7 × 8}$ = $\dfrac{64}{56}$

343.

a, $\dfrac{7}{6}$ và $\dfrac{6}{10}$

$\dfrac{7}{6}$ = $\dfrac{7 × 5}{6 × 5}$ = $\dfrac{35}{30}$

$\dfrac{6}{10}$ = $\dfrac{6 × 3}{10 × 3}$ = $\dfrac{18}{30}$

b, $\dfrac{19}{10}$ và $\dfrac{8}{9}$

$\dfrac{19}{10}$ = $\dfrac{19 × 9}{10 × 9}$ = $\dfrac{171}{90}$

$\dfrac{8}{9}$ = $\dfrac{8 × 10}{9 × 10}$ = $\dfrac{80}{90}$

344.

a, $\dfrac{41}{8}$ và $\dfrac{5}{4}$

$\dfrac{5}{4}$ = $\dfrac{5 × 2}{4 × 2}$ = $\dfrac{10}{8}$

b, $\dfrac{7}{10}$ và $\dfrac{83}{20}$

$\dfrac{7}{10}$ = $\dfrac{7 × 2}{10 × 2}$ = $\dfrac{14}{20}$

c, $\dfrac{19}{24}$v và $\dfrac{3}{8}$

$\dfrac{3}{8}$ = $\dfrac{3 × 3}{8 × 3}$ = $\dfrac{9}{24}$

345.

5 $\dfrac{7}{6}$ = $\dfrac{5 × 6 + 7}{6}$ = $\dfrac{37}{6}$

                          = $\dfrac{5 × 6 + 7}{6}$ = $\dfrac{37}{6}$ = $\dfrac{37 × 3}{6 × 3}$ = $\dfrac{111}{18}$