a: \(\dfrac{-4}{7}+\left(-\dfrac{2}{3}+\dfrac{4}{7}\right)\)
\(=-\dfrac{4}{7}-\dfrac{2}{3}+\dfrac{4}{7}\)
\(=-\dfrac{2}{3}\)
b: \(\dfrac{-3}{7}\cdot\dfrac{5}{9}+\dfrac{4}{9}\cdot\dfrac{-3}{7}+2\dfrac{3}{7}\)
\(=\dfrac{-3}{7}\left(\dfrac{5}{9}+\dfrac{4}{9}\right)+2+\dfrac{3}{7}\)
\(=-\dfrac{3}{7}+\dfrac{3}{7}+2\)
=2
c: \(\left(\dfrac{4}{5}+\dfrac{2}{3}\right):\dfrac{1}{5}-1\dfrac{2}{5}\cdot\left(-\dfrac{5}{7}\right)^2\)
\(=\dfrac{12+10}{15}\cdot\dfrac{5}{1}-\dfrac{7}{5}\cdot\dfrac{25}{49}\)
\(=\dfrac{22}{3}-\dfrac{7}{49}\cdot\dfrac{25}{5}\)
\(=\dfrac{22}{3}-\dfrac{5}{7}=\dfrac{139}{21}\)
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