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\(\dfrac{181818}{212121}=\dfrac{181818:10101}{212121:10101}=\dfrac{18}{21}\)
\(\dfrac{18}{21}=\dfrac{18:3}{21:3}=\dfrac{6}{7}\)
Vậy \(\dfrac{181818}{212121}=\dfrac{6}{7}\) ta chọn C
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\(\dfrac{3}{4}=\dfrac{12}{16};\dfrac{5}{6}>\dfrac{7}{9};\dfrac{4}{15}< \dfrac{4}{13};\dfrac{5}{4}-\dfrac{5}{6}>\dfrac{3}{4}-\dfrac{2}{3}\)
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\(a,\dfrac{x}{4}=\dfrac{15}{20}\\ \Rightarrow\dfrac{x}{4}=\dfrac{3}{4}\\ \Rightarrow x=3\\ b,\dfrac{15}{x}=\dfrac{25}{35}\\ \Rightarrow\dfrac{15}{x}=\dfrac{5}{7}\\ \Rightarrow\dfrac{15}{x}=\dfrac{15}{21}\\ \Rightarrow x=21\\ c,\dfrac{x}{5}=\dfrac{26}{65}\\ \Rightarrow\dfrac{x}{5}=\dfrac{2}{5}\\ \Rightarrow x=2\\ d,\dfrac{3}{x}=\dfrac{51}{85}\\ \Rightarrow\dfrac{3}{x}=\dfrac{3}{5}\\ \Rightarrow x=5\)
a,x4=1520⇒x4=34⇒x=3b,15x=2535⇒15x=57⇒15x=1521⇒x=21c,x5=2665⇒x5=25⇒x=2d,3x=5185⇒3x=35⇒x=5
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