Bài 14
a) \(\dfrac{-3}{7}:\dfrac{9}{14}=\dfrac{-3}{7}\cdot\dfrac{14}{9}=-\dfrac{2}{3}\)
b) \(\dfrac{-5}{8}\cdot\dfrac{12}{7}+\dfrac{-5}{8}\cdot\dfrac{2}{7}=\dfrac{-5}{8}\cdot\left(\dfrac{12}{7}+\dfrac{2}{7}\right)=\dfrac{-5}{8}\cdot\dfrac{14}{7}=\dfrac{-5}{8}\cdot2=\dfrac{-10}{8}=-\dfrac{5}{4}\)
c) \(75\%-2\dfrac{3}{4}+0,5\cdot\dfrac{2}{3}=\dfrac{3}{4}-\dfrac{11}{4}+\dfrac{1}{2}\cdot\dfrac{2}{3}=\left(-2\right)-\dfrac{1}{3}=-\dfrac{7}{3}\)
Bài 15 :
a) \(\dfrac{5}{2}\cdot x=1\dfrac{2}{3}\)
\(\dfrac{5}{2}\cdot x=\dfrac{5}{3}\)
\(x=\dfrac{5}{3}:\dfrac{5}{2}\)
\(x=\dfrac{5}{3}\cdot\dfrac{2}{5}\)
\(x=\dfrac{2}{3}\)
b) \(\dfrac{1}{3}\cdot x-50\%=\dfrac{-2}{5}\)
\(\dfrac{1}{3}\cdot x-\dfrac{1}{2}=\dfrac{-2}{5}\)
\(\dfrac{1}{3}\cdot x=\dfrac{-2}{5}+\dfrac{1}{2}\)
\(\dfrac{1}{3}\cdot x=\dfrac{1}{10}\)
\(x=\dfrac{1}{10}:\dfrac{1}{3}\)
\(x=\dfrac{1}{10}\cdot3\)
\(x=\dfrac{3}{10}\)
Sửa câu c) \(75\%-2\dfrac{3}{4}+0,5\cdot\dfrac{2}{3}=\dfrac{3}{4}-\dfrac{11}{4}+\dfrac{1}{2}\cdot\dfrac{2}{3}=\left(-2\right)+\dfrac{1}{3}=\dfrac{\left(-6\right)+1}{3}=-\dfrac{5}{3}\)
giải giúp tui với ạ sắp đến h đi hc rùi cảm ơn nhìu 
