a) $\dfrac{8}{-10}=\dfrac{4}{-5}$
⇒ $\dfrac{4}{-5}=\dfrac{-4}{5}$
⇒ $\dfrac{8}{-10}=\dfrac{-4}{5}$
b) $\dfrac{2}{3}=\dfrac{2.4}{3.4}=\dfrac{8}{12}$
$\dfrac{3}{4}=\dfrac{3.3}{4.3}=\dfrac{9}{12}$
⇒ $8<9$
⇒ $\dfrac{8}{12}<\dfrac{9}{12}$ hay $\dfrac{2}{3}<\dfrac{3}{4}$