a) $\dfrac{x}{5}=\dfrac{5}{6}+\dfrac{-19}{30}$;

b) $\dfrac{x}{15}=\dfrac{3}{5}+\dfrac{-2}{3}$.

a) $A=\dfrac{-5}{7}+\dfrac{3}{4}+\dfrac{-1}{5}+\dfrac{-2}{7}+\dfrac{1}{4}$;

b) $B=-\dfrac{3}{31}+\dfrac{-6}{17}+\dfrac{1}{25}+\dfrac{-28}{31}+\dfrac{-11}{17}+\dfrac{-1}{5}$;

c) $C=\dfrac{-1}{3}+\dfrac{2}{5}+\dfrac{-2}{3}+\dfrac{-3}{5}+\dfrac{1}{5}$.

a) $H=\dfrac{15}{2}-\Big(\dfrac{3}{5}+\dfrac{7}{2}\Big)+\dfrac{-2}{5}$ và $G=\dfrac{3}{4}+\Big(\dfrac{-7}{15}-\dfrac{-1}{4}\Big)+\dfrac{8}{-15}$.

b) $A=\dfrac{-10}{3}+\dfrac{6}{7}-\dfrac{2}{3}-\dfrac{-8}{7}$ và $B=\dfrac{21}{2}+\dfrac{2}{15}-\dfrac{9}{2}+\dfrac{-17}{15}$.