\(A=\dfrac{3}{7}\cdot\dfrac{-7}{19}+\dfrac{3}{5}\cdot\dfrac{5}{12}\cdot\dfrac{6}{19}-\dfrac{5}{19}\cdot\dfrac{6}{10}\\ =\dfrac{3}{7}\cdot\dfrac{-7}{19}+\dfrac{3}{5}\cdot\dfrac{5}{19}\cdot\dfrac{6}{12}-\dfrac{5}{19}\cdot\dfrac{6}{10}\\ =\dfrac{3}{7}\cdot\dfrac{-7}{19}+\dfrac{3}{5}\cdot\dfrac{5}{19}\cdot\dfrac{1}{2}-\dfrac{5}{19}\cdot\dfrac{3}{5}\\ =-\dfrac{3}{19}+\dfrac{3}{5}\cdot\left(\dfrac{5}{19}\cdot\dfrac{1}{2}-\dfrac{5}{19}\right)\\ =-\dfrac{3}{19}+\dfrac{3}{5}\cdot\dfrac{5}{19}\cdot\left(\dfrac{1}{2}-1\right)\\ =-\dfrac{3}{19}+\dfrac{3}{19}\cdot\dfrac{-1}{2}=\dfrac{3}{19}\cdot\left(-1+\dfrac{-1}{2}\right)\\ =\dfrac{3}{19}\cdot\dfrac{-3}{2}=-\dfrac{9}{38}\)

\(A=\dfrac{\dfrac{1}{3}-\dfrac{1}{7}-\dfrac{1}{13}}{\dfrac{2}{3}-\dfrac{2}{7}-\dfrac{2}{13}}\cdot\dfrac{\dfrac{3}{4}-\dfrac{3}{16}-\dfrac{3}{64}-\dfrac{3}{256}}{1-\dfrac{1}{4}-\dfrac{1}{16}-\dfrac{1}{64}}+\dfrac{5}{8}\\ =\dfrac{\dfrac{1}{3}-\dfrac{1}{7}-\dfrac{1}{13}}{2\left(\dfrac{1}{3}-\dfrac{1}{7}-\dfrac{1}{13}\right)}\cdot\dfrac{\dfrac{3}{4}\left(1-\dfrac{1}{4}-\dfrac{1}{16}-\dfrac{1}{64}\right)}{1-\dfrac{1}{4}-\dfrac{1}{16}-\dfrac{1}{64}}+\dfrac{5}{8}\\ =\dfrac{1}{2}\cdot\dfrac{3}{4}:1+\dfrac{5}{8}=\dfrac{1}{2}\cdot\dfrac{3}{4}+\dfrac{5}{8}=\dfrac{3}{8}+\dfrac{5}{8}=1\)

:DD......... thoy lỡ r