a: \(A=\left(\dfrac{2}{7}\cdot\dfrac{1}{4}-\dfrac{1}{3}\cdot\dfrac{2}{7}\right):\left(\dfrac{2}{7}\cdot\dfrac{3}{9}-\dfrac{2}{7}\cdot\dfrac{2}{5}\right)\)
\(=\dfrac{\dfrac{2}{7}\left(\dfrac{1}{4}-\dfrac{1}{3}\right)}{\dfrac{2}{7}\left(\dfrac{3}{9}-\dfrac{2}{5}\right)}=\dfrac{\dfrac{1}{4}-\dfrac{1}{3}}{\dfrac{3}{9}-\dfrac{2}{5}}\)
\(=\dfrac{-1}{12}:\left(\dfrac{15}{45}-\dfrac{18}{45}\right)=\dfrac{-1}{12}:\left(-\dfrac{3}{45}\right)\)
\(=\dfrac{1}{12}\cdot\dfrac{45}{3}=\dfrac{1}{12}\cdot15=\dfrac{15}{12}=\dfrac{5}{4}\)
b: \(B=\dfrac{\left(\dfrac{1}{5}-\dfrac{2}{7}\right)\cdot\dfrac{3}{4}-\dfrac{3}{4}\left(\dfrac{1}{3}-\dfrac{2}{7}\right)}{\dfrac{1}{5}\cdot\dfrac{2}{7}-\dfrac{1}{3}\left(\dfrac{2}{7}+\dfrac{3}{9}\right)+\dfrac{3}{9}\cdot\dfrac{1}{5}}\)
\(=\dfrac{\dfrac{3}{4}\left(\dfrac{1}{5}-\dfrac{2}{7}-\dfrac{1}{3}+\dfrac{2}{7}\right)}{\dfrac{1}{5}\left(\dfrac{2}{7}+\dfrac{1}{3}\right)-\dfrac{1}{3}\left(\dfrac{2}{7}+\dfrac{1}{3}\right)}=\dfrac{\dfrac{3}{4}\cdot\left(\dfrac{1}{5}-\dfrac{1}{3}\right)}{\left(\dfrac{2}{7}+\dfrac{1}{3}\right)\cdot\left(\dfrac{1}{5}-\dfrac{1}{3}\right)}\)
\(=\dfrac{3}{4}:\left(\dfrac{6}{21}+\dfrac{7}{21}\right)=\dfrac{3}{4}:\dfrac{13}{21}=\dfrac{3}{4}\cdot\dfrac{21}{13}=\dfrac{63}{52}\)
bài 2 và bài 3 thôi ạ