*)Nhận xét:\(\dfrac{2}{3}\times\dfrac{4}{5}=\,...;\) \(\dfrac{4}{5}\times\dfrac{2}{3}=\,...\) . Vậy\(\dfrac{2}{3}\times\dfrac{4}{5}\,...\, \dfrac{4}{5}\times\dfrac{2}{3}\) *)Nhận xét:\((\dfrac{1}{3}\times\dfrac{2}{5})\times\dfrac{3}{4}=\,...;\) \(\dfrac{1}{3}\times(\dfrac{2}{5}\times\dfrac{3}{4})=\,...\) .Vậy\((\dfrac{1}{3}\times\dfrac{2}{5})\times\dfrac{3}{4}\,...\dfrac{1}{3}\times(\dfrac{2}{5}\times\dfrac{3}{4})\) *)Nhận xét:\((\dfrac{1}{5}+\dfrac{2}{5})\times\dfrac{3}{4}=\,...;\) \(\dfrac{1}{5}\times\dfrac{3}{4}+\dfrac{2}{5}\times\dfrac{3}{4}=\,...\) .Vậy\((\dfrac{1}{5}+\dfrac{2}{5})\times\dfrac{3}{4}\,...\dfrac{1}{5}\times\dfrac{3}{4}+\dfrac{2}{5}\times\dfrac{3}{4}\) \(\dfrac{3}{22}\times\dfrac{3}{11}\times22;\) \((\dfrac{1}{2}+\dfrac{1}{3})\times\dfrac{2}{5};\) \(\dfrac{3}{5}\times\dfrac{17}{21}+\dfrac{17}{21}\times\dfrac{2}{5}\) a) *)Nhận xét:\(\dfrac{2}{3}\times\dfrac{4}{5}= \dfrac{2\,\times\,4}{3\,\times\,5\,}=\dfrac{8}{15}\) \(\dfrac{4}{5}\times\dfrac{2}{3}=\dfrac{4\,\times\,2}{5\,\times\,3\,}=\dfrac{8}{15}\) .Vậy\(\dfrac{2}{3}\times\dfrac{4}{5}\,=\,\dfrac{4}{5}\times\dfrac{2}{3}\)
*)Nhận xét:\((\dfrac{1}{3}\times\dfrac{2}{5})\times\dfrac{3}{4}=\dfrac{2}{15}\times\dfrac{3}{4}=\dfrac{6}{60}=\dfrac{1}{10}\) \(\dfrac{1}{3}\times(\dfrac{2}{5}\times\dfrac{3}{4})=\dfrac{1}{3}\times\dfrac{6}{20}=\dfrac{6}{60}=\dfrac{1}{10}\) .Vậy\((\dfrac{1}{3}\times\dfrac{2}{5})\times\dfrac{3}{4}\,=\dfrac{1}{3}\times(\dfrac{2}{5}\times\dfrac{3}{4})\)
*)Nhận xét:\((\dfrac{1}{5}+\dfrac{2}{5})\times\dfrac{3}{4}=\dfrac{3}{5}\times\dfrac{3}{4}=\dfrac{9}{20}\) \(\dfrac{1}{5}\times\dfrac{3}{4}+\dfrac{2}{5}\times\dfrac{3}{4}=\dfrac{3}{20}+\dfrac{6}{20}=\dfrac{9}{20}\) .Vậy\((\dfrac{1}{5}+\dfrac{2}{5})\times\dfrac{3}{4}\,=\dfrac{1}{5}\times\dfrac{3}{4}+\dfrac{2}{5}\times\dfrac{3}{4}\)
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