a) \(\frac{-1}{21}+\frac{-1}{28}-\frac{-1}{21}-\frac{3}{14}\)
\(=\frac{-1}{21}+\frac{-1}{28}+\frac{1}{21}+\frac{-3}{14}\)
\(=\left(\frac{-1}{21}+\frac{1}{21}\right)+\left(\frac{-1}{28}+\frac{-3}{14}\right)\)
\(=0+\left(\frac{-1}{28}+\frac{-6}{28}\right)\)
\(=0+\frac{-1}{4}=\frac{-1}{4}\)
b) \(\left(\frac{-1}{5}+\frac{3}{12}\right)+\frac{-3}{4}\)
\(=\frac{-1}{5}+\frac{1}{4}+\frac{-3}{4}\)
\(=\left(\frac{1}{4}+\frac{-3}{4}\right)+\frac{-1}{5}\)
\(=\frac{-1}{2}+\frac{-1}{5}\)
\(=\frac{-5}{10}+\frac{-2}{10}=\frac{-7}{10}\)
c) \(\frac{-4}{11}\cdot\frac{2}{5}+\frac{6}{11}\cdot\frac{-3}{10}\)
\(=\frac{-8}{55}+\frac{-9}{55}=\frac{-17}{55}\)
