a) \(1\dfrac{1}{5}+\dfrac{5}{9}+\dfrac{4}{5}+\dfrac{4}{9}\)
\(=\left(\dfrac{6}{5}+\dfrac{4}{5}\right)+\left(\dfrac{5}{9}+\dfrac{4}{9}\right)\\ =2+1\\ =3\)
b) \(2\dfrac{-7}{10}:\left(\dfrac{5}{7}+\dfrac{3}{14}\right)\)
\(=-\dfrac{27}{10}:\dfrac{13}{14}\\ =-\dfrac{27}{10}\cdot\dfrac{14}{13}\\ =-\dfrac{189}{65}\)
c) \(\dfrac{1}{12}+\dfrac{1}{20}+\dfrac{1}{30}+\dfrac{1}{42}+\dfrac{1}{56}+\dfrac{1}{72}+\dfrac{1}{90}\)
\(=\dfrac{1}{3\cdot4}+\dfrac{1}{4\cdot5}+\dfrac{1}{5\cdot6}+\dfrac{1}{6\cdot7}+\dfrac{1}{7\cdot8}+\dfrac{1}{8\cdot9}+\dfrac{1}{9\cdot10}\)
\(=\dfrac{1}{3}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{6}+\dfrac{1}{6}-\dfrac{1}{7}+\dfrac{1}{7}-\dfrac{1}{8}+\dfrac{1}{8}-\dfrac{1}{9}+\dfrac{1}{9}-\dfrac{1}{10}\)
\(=\dfrac{1}{3}-\dfrac{1}{10}\\ =\dfrac{7}{30}\)
