\(a,\dfrac{1}{2}-2xy=\dfrac{9}{20}\\ 2xy=\dfrac{1}{2}-\dfrac{9}{20}\\ 2xy=\dfrac{10}{20}-\dfrac{9}{20}\\ 2xy=\dfrac{1}{20}\\ xy=\dfrac{1}{20}:2\\ xy=\dfrac{1}{20}\times\dfrac{1}{2}\\ xy=\dfrac{1}{40}\\ b,\dfrac{3}{5}:\dfrac{4}{3}:y=2+\dfrac{7}{10}\\ \dfrac{9}{20}:y=\dfrac{20}{10}+\dfrac{7}{10}\\ \dfrac{9}{20}:y=\dfrac{27}{10}\\ y=\dfrac{9}{20}:\dfrac{27}{10}\\ y=\dfrac{9}{20}\times\dfrac{10}{27}\\ x=\dfrac{90}{540}\\ x=\dfrac{1}{6}\)
\(c,y+y\times\dfrac{3}{2}-y\times\dfrac{1}{2}=\dfrac{1}{10}\\ y\times1+y\times\dfrac{3}{2}-y\times\dfrac{1}{2}=\dfrac{1}{10}\\ y\times\left(1+\dfrac{3}{2}-\dfrac{1}{2}\right)=\dfrac{1}{10}\\ y\times\left(\dfrac{2}{2}+\dfrac{3}{2}-\dfrac{1}{2}\right)=\dfrac{1}{10}\\ y\times2=\dfrac{1}{10}\\ y=\dfrac{1}{10}:2\\ y=\dfrac{1}{10}\times\dfrac{1}{2}\\ y=\dfrac{1}{20}\)
4 và 5
