`5/9 xx a/b -11/21 =4/21`
`=> 5/9 xx a/b = 4/21 + 11/21`
`=> 5/9 xx a/b=15/21`
`=> a/b= 15/21 : 5/9`
`=>a/b= 15/21 xx 9/5`
`=> a/b=135/105`
`=>a/b=9/7`
Vậy `a/b=9/7`
\(\dfrac{5}{9}\times\dfrac{a}{b}-\dfrac{11}{21}=\dfrac{4}{21}\)
\(\Leftrightarrow\dfrac{5}{9}\times\dfrac{a}{b}=\dfrac{15}{21}\)
\(\Leftrightarrow\dfrac{a}{b}\times\dfrac{5}{9}=\dfrac{15}{21}\)
\(\Leftrightarrow\dfrac{a}{b}=\dfrac{15}{21}:\dfrac{5}{9}\)
\(\Leftrightarrow\dfrac{a}{b}=\dfrac{135}{105}=\dfrac{9}{7}\)
vaayjj : \(\dfrac{a}{b}=\dfrac{9}{7}\)
\(\dfrac{5}{9}\times\dfrac{a}{b}-\dfrac{11}{21}=\dfrac{4}{21}\\ \Rightarrow\dfrac{5}{9}\times\dfrac{a}{b}=\dfrac{4}{21}+\dfrac{11}{21}\\ \Rightarrow\dfrac{5}{9}\times\dfrac{a}{b}=\dfrac{5}{7}\\ \Rightarrow\dfrac{a}{b}=\dfrac{5}{7}\div\dfrac{5}{9}\\ \Rightarrow\dfrac{a}{b}=\dfrac{5}{7}\times\dfrac{9}{5}\\ \Rightarrow\dfrac{a}{b}=\dfrac{9}{7}\)
Vậy \(\dfrac{a}{b}=\dfrac{9}{7}\)
