2/7 + 4/9 + 5/7 = ( 2/7 + 5/7) + 4/9 = 1 + 4/9 = 13/9
35,72 + ( 15,17 + 84,83) = 35,72 + 100 = 135,72
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Bài 3:
\(a.\dfrac{2}{7}+\left(\dfrac{4}{9}+\dfrac{5}{7}\right)\)
\(=\left(\dfrac{2}{7}+\dfrac{5}{7}\right)+\dfrac{4}{9}\)
\(=1+\dfrac{4}{9}=\dfrac{13}{9}\)
\(b.35,72+15,17+84,83\)
\(=35,72+\left(15,17+84,83\right)\)
\(=35,72+100=135,72\)
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