Tính nhanh.
Bài 6. Tính nhanh.
a) $\frac{12}{23}\times\frac{7}{13}+\frac{11}{23}\times\frac{7}{13}$
b) $\frac{4}{9}\times\frac{23}{11}-\frac{1}{11}\times\frac{4}{9}+\frac{4}{9}$
c) $[(-\frac{5}{7})+\frac{3}{5}]/\frac{2020}{2021}+(\frac{2}{5}-\frac{2}{7})/\frac{2020}{2021}$
d) $\frac{3}{8}/(\frac{7}{22}-\frac{2}{11})+\frac{3}{8}/(\frac{2}{5}-\frac{1}{10})$
a) $\frac{12}{23}\times\frac{7}{13}+\frac{11}{23}\times\frac{7}{13}=\frac{7}{13}\times (\frac{12}{23}+\frac{11}{23})=\frac{7}{13}$
b) $\frac{4}{9}\times\frac{23}{11}-\frac{1}{11}\times\frac{4}{9}+\frac{4}{9}=\frac{4}{9}\times (\frac{23}{11}-\frac{1}{11}+1)=\frac{4}{9}\times 3=\frac{4}{3}$
c) $[(-\frac{5}{7})+\frac{3}{5}]/\frac{2020}{2021}+(\frac{2}{5}-\frac{2}{7})/\frac{2020}{2021}=[(\frac{-5}{7})+\frac{3}{5}+\frac{2}{5}+(\frac{-2}{7})]/\frac{2020}{2021}=0$
d) $\frac{3}{8}/(\frac{7}{22}-\frac{2}{11})+\frac{3}{8}/(\frac{2}{5}-\frac{1}{10})=\frac{3}{8}/\frac{3}{22}+\frac{3}{8}/\frac{3}{10}=\frac{3}{8}\times (\frac{22}{3}+\frac{10}{3})=4$
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