a: \(\dfrac{5}{-9}=\dfrac{15}{-27};\dfrac{5}{15}=\dfrac{-9}{-27};\dfrac{-9}{5}=\dfrac{-27}{15};\dfrac{15}{5}=\dfrac{-27}{-9}\)
d: \(\dfrac{12}{15}=\dfrac{16}{20};\dfrac{12}{16}=\dfrac{15}{20};\dfrac{15}{12}=\dfrac{20}{16};\dfrac{16}{12}=\dfrac{20}{15}\)