\(=\dfrac{1}{2}-\dfrac{2}{5}-\dfrac{21}{10}=\dfrac{5}{10}-\dfrac{4}{5}-\dfrac{21}{10}=-\dfrac{20}{10}=-2\)
\(\dfrac{1}{2}-\dfrac{2}{5}+\dfrac{-21}{10}=\dfrac{5}{10}-\dfrac{4}{10}+\dfrac{-21}{10}=\dfrac{5-4+\left(-21\right)}{10}=\dfrac{-20}{10}=-2\)
\(\dfrac{1}{2}-\dfrac{2}{5}+\dfrac{-7}{10}\cdot3=\dfrac{1}{2}-\dfrac{2}{5}+\dfrac{-21}{10}=\dfrac{5}{10}-\dfrac{4}{10}+\dfrac{-21}{10}=\dfrac{1}{10}+\dfrac{-21}{10}=\dfrac{-20}{10}=-2\)
\(\dfrac{1}{2}-\dfrac{2}{5}-\dfrac{21}{10}\)
\(=\dfrac{5}{10}-\dfrac{4}{10}-\dfrac{21}{10}=-\dfrac{20}{10}=-2\)