a: \(\dfrac{2}{5}+\dfrac{3}{5}\cdot\dfrac{-10}{21}\)
\(=\dfrac{2}{5}+\dfrac{-30}{105}\)
\(=\dfrac{2}{5}-\dfrac{2}{7}=\dfrac{14-10}{35}=\dfrac{4}{35}\)
b: \(\dfrac{-4}{5}:\dfrac{2}{7}+\dfrac{4}{7}\)
\(=\dfrac{-4}{5}\cdot\dfrac{7}{2}+\dfrac{4}{7}\)
\(=\dfrac{-14}{5}+\dfrac{4}{7}=\dfrac{-98+20}{35}=\dfrac{-78}{35}\)
c: \(\dfrac{21}{25}\cdot\dfrac{11}{9}\cdot\dfrac{5}{7}\)
\(=\dfrac{21}{7}\cdot\dfrac{5}{25}\cdot\dfrac{11}{9}=\dfrac{3}{5}\cdot\dfrac{11}{9}=\dfrac{3}{9}\cdot\dfrac{11}{5}=\dfrac{11}{15}\)
d: \(\left(-\dfrac{2}{5}+\dfrac{1}{3}\right)\left(\dfrac{3}{2}-\dfrac{3}{7}\right)\)
\(=\dfrac{-6+5}{15}\cdot\dfrac{21-6}{14}\)
\(=\dfrac{-1}{15}\cdot\dfrac{15}{14}=-\dfrac{1}{14}\)
e: \(\left(\dfrac{1}{2}-\dfrac{1}{3}\right)\left(5-\dfrac{1}{4}\right)\)
\(=\dfrac{3-2}{6}\cdot\dfrac{20-1}{4}\)
\(=\dfrac{1}{6}\cdot\dfrac{19}{4}=\dfrac{19}{24}\)
f: \(\left(\dfrac{4}{5}\right)^2=\dfrac{4^2}{5^2}=\dfrac{16}{25}\)
