a) Ta có : $\dfrac{-21}{1.2} = \dfrac{-21}{2}$
$\dfrac{-15}{2.13} = \dfrac{-2-13}{2.13} = \dfrac{-1}{13}-\dfrac{1}{2}$
$\dfrac{-9}{13.4} = \dfrac{4-13}{13.4}=\dfrac{1}{13}-\dfrac{1}{4} $
$\dfrac{-3}{4.17}=\dfrac{17-5.4}{4.17} = \dfrac{1}{4}-\dfrac{5}{17} $
$\dfrac{-33}{17.7} = \dfrac{35-68}{17.7} = \dfrac{5}{17}-\dfrac{4}{7} $
Khi đó biểu thức cần tính còn :
$\dfrac{-21}{2} - \dfrac{1}{2}- \dfrac{4}{7} = \dfrac{-81}{7}$
