Câu 11
a) \(\dfrac{-5}{6}+\dfrac{3}{-7}\)
\(=\dfrac{-5}{6}+\dfrac{-3}{7}\)
\(=-\dfrac{53}{42}\)
b) \(\dfrac{2}{5}-\dfrac{4}{9}\)
\(=\dfrac{18}{45}-\dfrac{20}{45}\)
\(=-\dfrac{2}{45}\)
c) \(\dfrac{-6}{5}\cdot\dfrac{4}{7}\)
\(=-\dfrac{24}{35}\)
d) \(\dfrac{2}{3}\div\dfrac{-4}{5}\)
\(=\dfrac{2}{3}\cdot\dfrac{-5}{4}\)
\(=-\dfrac{10}{12}=-\dfrac{5}{6}\)
Câu 12
a) \(\dfrac{14}{13}+\left(\dfrac{-1}{13}-\dfrac{19}{20}\right)\)
\(=\left(\dfrac{14}{13}+\dfrac{-1}{13}\right)-\dfrac{19}{20}\)
\(=1-\dfrac{19}{20}=\dfrac{1}{20}\)
b) \(\dfrac{-24}{17}-\left(\dfrac{-7}{17}-\dfrac{1}{16}\right)\)
\(=\left(\dfrac{-24}{17}-\dfrac{-7}{17}\right)-\dfrac{1}{16}\)
\(=\left(-1\right)-\dfrac{1}{16}\)
\(=-\dfrac{17}{16}\)
Câu 13
a) \(\dfrac{4}{7}\cdot x-\dfrac{2}{3}=\dfrac{1}{5}\)
\(\dfrac{4}{7}\cdot x=\dfrac{1}{5}+\dfrac{2}{3}\)
\(\dfrac{4}{7}\cdot x=\dfrac{13}{15}\)
\(x=\dfrac{13}{15}\div\dfrac{4}{7}\)
\(x=\dfrac{91}{60}\)
b) \(\dfrac{4}{5}+\dfrac{5}{7}\div x=\dfrac{1}{6}\)
\(\dfrac{5}{7}\div x=\dfrac{4}{5}-\dfrac{1}{6}\)
\(\dfrac{5}{7}\div x=-\dfrac{19}{30}\)
\(x=\dfrac{5}{7}\div\left(-\dfrac{19}{30}\right)\)
\(x=-\dfrac{150}{133}\)