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a)\(\dfrac{5}{7}+\dfrac{4}{9}=\dfrac{45}{63}+\dfrac{28}{63}=\dfrac{73}{63}\) ; \(\dfrac{9}{11}+\dfrac{3}{8}=\dfrac{72}{88}+\dfrac{33}{88}=\dfrac{105}{88}\)
\(\dfrac{4}{5}-\dfrac{2}{3}=\dfrac{12}{15}-\dfrac{10}{15}=\dfrac{2}{15}\); \(\dfrac{16}{25}-\dfrac{2}{5}=\dfrac{16}{25}-\dfrac{10}{25}=\dfrac{6}{25}\)
b)\(5+\dfrac{3}{5}=\dfrac{25}{5}+\dfrac{3}{5}=\dfrac{28}{5};10-\dfrac{9}{16}=\dfrac{160}{16}-\dfrac{9}{16}=\dfrac{151}{16}\)
\(\dfrac{2}{3}-\left(\dfrac{1}{6}+\dfrac{1}{8}\right)=\dfrac{2}{3}-\left(\dfrac{8}{48}+\dfrac{6}{48}\right)=\dfrac{2}{3}-\dfrac{14}{48}=\dfrac{32}{48}-\dfrac{14}{48}=\dfrac{3}{8}\)
Quy đồng :
\(a,\frac{4}{5},\frac{7}{9}\)
\(b,\frac{5}{6},\frac{17}{18}\)
\(c,\frac{3}{8},\frac{7}{12}\)
A ) 4 / 5 = 4 x 9 / 5 x 9 = 36 / 45
7 / 9 = 7 x 5 / 9 x 5 = 35/ 45
B ) 5 / 6 = 5 x 18 / 6 x 18 = 90 / 108
17 / 18 = 17 x 6 / 18 x 6 = 102 / 108
C ) 3 / 8 = 3 x 12 / 8 x 12 = 36 / 96
7 / 12 = 7 x 8 / 12 x 8 = 56 / 96
Chọn B