Giải bài 2 trang 167 - SGK Toán lớp 4

a) $\dfrac{2}{7}+ \dfrac{3}{5}=\dfrac{10}{35}+ \dfrac{21}{35}=\dfrac{31}{35}$             

$\dfrac{31}{35}-\dfrac{2}{7}=\dfrac{31}{35}-\dfrac{10}{35}=\dfrac{21}{35}=\dfrac{3}{5}$                 

$\dfrac{31}{35}-\dfrac{3}{5}=\dfrac{31}{35}-\dfrac{21}{35}=\dfrac{10}{35}=\dfrac{2}{7}$       

$\dfrac{3}{5}+ \dfrac{2}{7}=\dfrac{21}{35}+ \dfrac{10}{35}=\dfrac{31}{35}$

b)

 $\dfrac{3}{4}+ \dfrac{1}{6}=\dfrac{9}{12}+ \dfrac{2}{12}=\dfrac{11}{12}$       

$\dfrac{11}{12}-\dfrac{3}{4} =\dfrac{11}{12}-\dfrac{9}{12}=\dfrac{2}{12}=\dfrac{1}{6}$           

$\dfrac{11}{12}-\dfrac{3}{6}=\dfrac{11}{12}-\dfrac{6}{12}=\dfrac{5}{12}$           

$\dfrac{1}{6}+ \dfrac{3}{4}=\dfrac{2}{12}+ \dfrac{9}{12}=\dfrac{11}{12}$