Giải bài 2 trang 169 - SGK Toán lớp 4

a) $\dfrac{2\,\times\,3\,\times\,4}{3\,\times\,4\,\times\,5}= \dfrac {{2} \,\times \cancel{3} \times \cancel{4} } {\cancel{3} \times \cancel{4} \times \,{5}} =\dfrac{2}{5}$

b) $\dfrac{2}{3}\times\dfrac{3}{4}\times\dfrac{4}{5}: \dfrac{1}{5}=\dfrac{2}{3}\times\dfrac{3}{4}\times\dfrac{4}{5}\times\dfrac{5}{1}=\dfrac{2\,\times\,3\,\times\,4\,\times\,5}{3\,\times\,4\,\times\,5\,\times\,1}= \dfrac {{2} \,\times \cancel{3} \times \cancel{4}\times\,\cancel5} {\cancel{3} \times \cancel{4} \times \,\cancel{5}\,\times\,1} =2$

c) $\dfrac{1\,\times\,2\,\times\,3\,\times\,4\,}{5\times\,6\,\times\,7\,\times\,8}=\dfrac{1}{70}$

d) $\dfrac{2}{5}\times\dfrac{3}{4}\times\dfrac{5}{6}: \dfrac{3}{4}=\dfrac{2}{5}\times\dfrac{3}{4}\times\dfrac{5}{6}\times\dfrac{4}{3}=\dfrac{2\,\times\,3\,\times\,5\,\times\,4}{5\,\times\,4\,\times\,6\,\times\,3}= \dfrac {\cancel{2} \,\times \cancel{3} \times \cancel{5}\times\,\cancel4} {\cancel{5} \times \cancel{4} \times \,\cancel{6}\,\times\,\cancel3} =\dfrac{1}{3}$