a: \(-\dfrac{8}{24}=\dfrac{-1.4}{4.2}\)
\(\Leftrightarrow\dfrac{-8}{-1.4}=\dfrac{24}{4.2};\dfrac{24}{-8}=\dfrac{4.2}{-1.4};\dfrac{-1.4}{-8}=\dfrac{4.2}{24}\)
b: \(\dfrac{42}{15}=\dfrac{2.8}{0.5}\)
\(\Leftrightarrow\dfrac{42}{2.8}=\dfrac{15}{0.5};\dfrac{15}{42}=\dfrac{0.5}{2.8};\dfrac{2.8}{42}=\dfrac{0.5}{15}\)