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Câu 7:
a: \(18+\dfrac{9}{3}=18+3=21\)
b: \(\dfrac{4}{9}-\dfrac{5}{18}=\dfrac{8}{18}-\dfrac{5}{18}=\dfrac{3}{18}=\dfrac{1}{6}\)
c: \(\dfrac{3}{5}:\dfrac{5}{7}=\dfrac{3}{5}\cdot\dfrac{7}{5}=\dfrac{21}{25}\)
d: \(\dfrac{4}{7}\cdot\dfrac{2}{7}=\dfrac{4\cdot2}{7\cdot7}=\dfrac{8}{49}\)
Câu 8:
a: \(\dfrac{5}{4}\cdot x=\dfrac{3}{8}+\dfrac{5}{12}\)
=>\(x\cdot\dfrac{5}{4}=\dfrac{9+10}{24}=\dfrac{19}{24}\)
=>\(x=\dfrac{19}{24}:\dfrac{5}{4}=\dfrac{19}{24}\cdot\dfrac{4}{5}=\dfrac{19}{6\cdot5}=\dfrac{19}{30}\)
b: \(3\cdot\dfrac{7}{10}+\dfrac{7}{10}\cdot5+2\cdot\dfrac{7}{10}\)
\(=\dfrac{7}{10}\left(3+5+2\right)\)
\(=\dfrac{7}{10}\cdot10=7\)
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16,2 * 3,7 + 5,7 * 16,2 -7,8 * 4,8 - 4,6 * 7,8
= 16,2 * (3,7 + 5,7) - 7,8 * (4,8 - 4,6)
=16,2 * 9,4 - 7,8 * 0,2
=58,28 - 1,56
=56,72
7. B
8. 3/4
B