Rút gọn các phân số:
\(\displaystyle\eqalign{ & {{21} \over 28} = \;...\,\,\,\,\,\,\,\,\,\,\quad \quad \quad \quad \quad \quad {9 \over {36}} =\; ... \cr & {{18} \over {54}} =\; ...\,\,\,\,\,\,\,\,\,\,\quad \quad \quad \quad \quad \quad{{30} \over {48}} =\; ... \cr & {{90} \over {72}} =\; ...\,\,\,\,\,\,\,\,\,\,\quad \quad \quad \quad \quad \quad{{72} \over {42}} =\; ... \cr} \)
\(\displaystyle {{21} \over 28} = \dfrac{21:7}{28:7}= {3 \over 4} \\\displaystyle{9 \over {36}}=\dfrac{9:9}{36 : 4} = {1 \over 4} \\\dfrac{18}{54}= \dfrac{18:18}{54:18}= \dfrac{1}{3} \\\displaystyle{{30} \over {48}} = \dfrac{30 : 6}{48 : 6}= {5 \over 8} \\\displaystyle {{90} \over {72}}= \dfrac{90:18}{72:18} = {5 \over 4} \\\displaystyle{{72} \over {42}} = \dfrac{72:6}{42:6}= {{12} \over 7} \)
-- Mod Toán lớp 4