Giải bài 2 trang 177 - SGK Toán lớp 4

a) $\dfrac{2}{5}+\dfrac{3}{10}- \dfrac{1}{2}=\dfrac{4}{10}+\dfrac{3}{10}- \dfrac{5}{10}= \dfrac{4 + 3 -5}{10}= \dfrac{2}{10}= \dfrac{1}{5}$;     

b ) $\dfrac{8}{11}+\dfrac{8}{33}\times\dfrac{3}{4}=\dfrac{8}{11}+\dfrac{2}{11}=\dfrac{10}{11}$

c) $\dfrac{7}{9}\times\dfrac{3}{14}:\dfrac{5}{8}=\dfrac{1}{6}\times\dfrac{8}{5}=\dfrac{8}{30}=\dfrac{4}{15}$;           

d) $\dfrac{5}{12}-\dfrac{7}{32}:\dfrac{21}{16}=\dfrac{5}{12}-\dfrac{7}{32}\times\dfrac{16}{21}=\dfrac{5}{12}-\dfrac{1}{6}=\dfrac{5}{12}-\dfrac{2}{12}=\dfrac{3}{12}=\dfrac{1}{4}$