\(a,\dfrac{5}{12}-\dfrac{3}{9}+\dfrac{1}{18}=\dfrac{15}{36}-\dfrac{12}{36}+\dfrac{2}{36}=\dfrac{3}{36}+\dfrac{2}{36}=\dfrac{5}{36}\\ b,\dfrac{-2}{5}-\dfrac{3}{5}.\dfrac{4}{9}=\dfrac{-2}{5}-\dfrac{12}{45}=\dfrac{-18}{45}-\dfrac{12}{45}=\dfrac{-30}{45}=\dfrac{-2}{3}\)

\(c,\dfrac{2}{9}-\dfrac{-5}{12}+\dfrac{-3}{4}=\dfrac{2}{9}+\dfrac{5}{12}+\dfrac{-3}{4}=\dfrac{8}{36}+\dfrac{15}{36}+\dfrac{-27}{36}=\dfrac{23}{36}+\dfrac{-27}{36}=\dfrac{-4}{36}=\dfrac{-1}{9}\)

\(d,\dfrac{7}{12}-\dfrac{27}{7}:\dfrac{-18}{7}=\dfrac{7}{12}-\dfrac{27}{7}.\dfrac{7}{-18}=\dfrac{7}{12}-\dfrac{-189}{126}=\dfrac{7}{12}+\dfrac{189}{126}=\dfrac{7}{12}+\dfrac{18}{12}=\dfrac{25}{12}\)

\(e,\left(\dfrac{7}{15}-\dfrac{11}{30}\right).\dfrac{-25}{3}=\left(\dfrac{14}{30}-\dfrac{11}{30}\right).\dfrac{-25}{3}=\dfrac{1}{10}.\dfrac{-25}{3}=\dfrac{-25}{30}=\dfrac{-5}{6}\)

\(f,\left(\dfrac{-5}{6}-\dfrac{1}{2}\right):\left(\dfrac{8}{15}-\dfrac{-3}{15}\right)=\left(\dfrac{-5}{6}-\dfrac{1}{2}\right):\left(\dfrac{8}{15}+\dfrac{3}{15}\right)=\left(\dfrac{-5}{6}-\dfrac{1}{2}\right):\dfrac{11}{15}=\left(\dfrac{-5}{6}-\dfrac{3}{6}\right):\dfrac{11}{15}=\dfrac{-4}{3}:\dfrac{11}{15}=\dfrac{-4}{3}.\dfrac{15}{11}=\dfrac{-60}{33}=\dfrac{-20}{11}\)

\(3km35m=3,035km\\ 42m^24dm^2=4204dm^2\\ 4tạ5kg=4,05tạ\\ 25m^28dm^2=25,08m^{2^{ }}\)

\(a,\dfrac{7}{9}=\dfrac{7.5}{9.5}=\dfrac{35}{45}\\\dfrac{2}{5}=\dfrac{2.9}{5.9}=\dfrac{18}{45}\\ b,\dfrac{6}{7}=\dfrac{6.3}{7.3}=\dfrac{18}{21}\\ \dfrac{17}{21}giữnguyên\\ c,\dfrac{2}{3}=\dfrac{2.12}{3.12}=\dfrac{24}{36}\\ \dfrac{1}{4}=\dfrac{1.9}{4.9}=\dfrac{9}{36}\\ \dfrac{5}{9}=\dfrac{5.4}{9.4}=\dfrac{20}{36}\)

1 tấn rưỡi = 1,5 tấn = 15 tạ = 150 yến = 1500 kg

\(a,30.25.7.8\\ =\left(30.7\right).\left(25.8\right)\\ =210.200\\ =42000\\ b,75.8.12.14\\ =\left(75.12\right).\left(8.14\right)\\ =900.112\\ =100800\)

\(\dfrac{56.5}{6}=\dfrac{280}{6}=\dfrac{140}{3}\)

\(\dfrac{3}{10}+\dfrac{5}{8}=\dfrac{12}{40}+\dfrac{25}{40}=\dfrac{12+25}{40}=\dfrac{37}{40}\)

\(4+\dfrac{9}{10}=\dfrac{40}{10}+\dfrac{9}{10}=\dfrac{40+9}{10}=\dfrac{49}{10}\)

\(a,\dfrac{6}{36}+\dfrac{5}{6}=\dfrac{1}{6}+\dfrac{5}{6}=\dfrac{6}{6}=1\\ b,\dfrac{1}{4}+\dfrac{5}{30}=\dfrac{1}{4}+\dfrac{1}{6}=\dfrac{6}{24}+\dfrac{4}{24}=\dfrac{5}{12}\\ c,\dfrac{5}{56}+\dfrac{3}{21}=\dfrac{5}{56}+\dfrac{8}{56}=\dfrac{13}{56}\\ d,\dfrac{12}{18}+\dfrac{12}{42}=\dfrac{14}{21}+\dfrac{6}{21}=\dfrac{20}{21}\)