Giải bài tập 2 trang 167 sgk toán 42.
$\frac{2}{7} + \frac{3}{5}= \frac{10}{35}+ \frac{21}{35}= \frac{31}{35}$
$\frac{31}{35} - \frac{2}{7}= \frac{31}{35}- \frac{10}{35}= \frac{21}{35}$
$\frac{31}{35} - \frac{3}{5}= \frac{31}{35}- \frac{21}{35}= \frac{10}{35}$
$\frac{3}{5} + \frac{2}{7}= \frac{21}{35}+ \frac{10}{35}= \frac{31}{35}$
$\frac{3}{4} + \frac{1}{6}$ $\frac{11}{12} - \frac{3}{4}$ $\frac{11}{12} - \frac{1}{6}$ $\frac{1}{6} + \frac{3}{4}$
$\frac{3}{4} + \frac{1}{6}= \frac{9}{12}+ \frac{2}{12}= \frac{11}{12}$
$\frac{11}{12} - \frac{3}{4}= \frac{11}{12}- \frac{9}{12}= \frac{2}{12}$
$\frac{11}{12} - \frac{1}{6}= \frac{11}{12}- \frac{2}{12}= \frac{9}{12}$
$\frac{1}{6} + \frac{3}{4}= \frac{2}{12}+ \frac{9}{12}= \frac{11}{12}$