Giải bài 1 trang 167 - SGK Toán lớp 4

a) $\dfrac{2}{7}+ \dfrac{4}{7} = \dfrac{2\,+\,4}{7}=\dfrac{6}{7}$                         $\dfrac{6}{7}-\dfrac{2}{7}= \dfrac{6\,-\,2}{7}=\dfrac{4}{7}$

    $\dfrac{6}{7}-\dfrac{4}{7}= \dfrac{6\,-\,4}{7}=\dfrac{2}{7}$                         $\dfrac{4}{7}+ \dfrac{2}{7}= \dfrac{4\,+\,2}{7}=\dfrac{6}{7}$

b) $\dfrac{1}{3}+ \dfrac{5}{12}=\dfrac{4}{12}+ \dfrac{5}{12}=\dfrac{9}{12}=\dfrac{3}{4}$         

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

    $\dfrac{9}{12}-\dfrac{5}{12}=\dfrac{9\,-\,5}{12}=\dfrac{2}{12}=\dfrac{1}{6}$         

   $\dfrac{5}{12}+ \dfrac{1}{3}=\dfrac{5}{12}+ \dfrac{4}{12}=\dfrac{9}{12}=\dfrac{3}{4}$