a, 15/25 - 3/21 = 3/5 - 1/7 = 21/35 - 5/35 = 18/35

b, 24/36 - 6/12 = 2/3 - 1/2 = 4/6 - 3/6 = 1/6

a)15/25-3/21=3/5-1/7=16/35                b)24/36-6/12=2/3-1/2=1/6

Cảm ơn

\(\dfrac{15}{25}=\dfrac{15:5}{25:5}=\dfrac{3}{5}\\ \dfrac{24}{28}=\dfrac{24:4}{28:4}=\dfrac{6}{7}\\ \dfrac{18}{33}=\dfrac{18:3}{33:3}=\dfrac{6}{11}\\ \dfrac{12}{36}=\dfrac{12:12}{36:12}=\dfrac{1}{3}.\)

\(\dfrac{15}{25}=\dfrac{3}{5}\)

\(\dfrac{24}{28}=\dfrac{6}{7}\)

\(\dfrac{18}{33}=\dfrac{6}{11}\)

\(\dfrac{12}{36}=\dfrac{1}{3}\)

\(\dfrac{15}{25}=\dfrac{15:5}{25:5}=\dfrac{3}{5}\\ \dfrac{24}{28}=\dfrac{24:4}{28:4}=\dfrac{6}{7}\\ \dfrac{18}{33}=\dfrac{18:3}{33:3}=\dfrac{6}{11}\\ \dfrac{12}{36}=\dfrac{12:12}{36:12}=\dfrac{1}{3}\)

Tính các tổng dưới đây sau khi đã rút gọn phân số :

a)\(\dfrac{7}{21}\) + \(\dfrac{9}{-36}\) = \(\dfrac{7}{21}\)+\(\dfrac{-9}{36}\)=\(\dfrac{1}{3}\)+\(\dfrac{-1}{4}\)=\(\dfrac{4}{12}\)+\(\dfrac{-3}{12}\)=\(\dfrac{1}{12}\)

b) \(\dfrac{-12}{18}\)+\(\dfrac{-21}{35}\)=\(\dfrac{-2}{3}\)+\(\dfrac{-3}{5}\)=\(\dfrac{-10}{15}\)+\(\dfrac{-9}{15}\)=\(\dfrac{-19}{15}\)

c) \(\dfrac{-3}{21}\)+\(\dfrac{6}{42}\)=\(\dfrac{-1}{7}\)+\(\dfrac{1}{7}\)=0

d) \(\dfrac{-18}{24}\)+\(\dfrac{15}{-21}\)=\(\dfrac{-18}{24}\)+\(\dfrac{-15}{21}\)=\(\dfrac{-3}{4}\)+\(\dfrac{-5}{7}\)=\(\dfrac{-21}{28}\)+\(\dfrac{-20}{28}\)=\(\dfrac{-41}{28}\)

\(a,\dfrac{1}{7}+\dfrac{1}{3}=\dfrac{3}{21}+\dfrac{7}{21}=\dfrac{10}{21}\\ b,\dfrac{-2}{3}+\dfrac{-5}{7}=\dfrac{-14+\left(-15\right)}{21}=\dfrac{-29}{21}\\ c,\dfrac{1}{2}-\dfrac{-1}{2}-\dfrac{1}{3}=\dfrac{3+3-2}{6}=\dfrac{4}{6}=\dfrac{2}{3}\)

\(a.\dfrac{4}{28}+\dfrac{12}{36}=\dfrac{1}{7}+\dfrac{1}{3}=\dfrac{3}{21}+\dfrac{7}{21}=\dfrac{10}{21}\\ b.\dfrac{-12}{18}+\dfrac{-15}{21}=\dfrac{-2}{3}+\dfrac{-5}{7}=\dfrac{-14}{21}+\dfrac{-15}{21}=\dfrac{-29}{21}\\ c.\dfrac{14}{28}+\dfrac{16}{32}-\dfrac{17}{51}=\dfrac{1}{2}+\dfrac{1}{2}-\dfrac{17}{51}=1-\dfrac{17}{51}=\dfrac{2}{3}\)

\(\dfrac{4}{28}+\dfrac{12}{36}=\dfrac{144}{1008}+\dfrac{336}{1008}=\\ \dfrac{480}{1008}=\dfrac{10}{21}\)

Câu b và c em làm tương tự.

a) \(\dfrac{2}{3}-\dfrac{2}{6}=\dfrac{4}{6}-\dfrac{2}{6}=\dfrac{4-2}{6}=\dfrac{2}{6}=\dfrac{1}{3}\)

b) \(\dfrac{5}{6}-\dfrac{3}{18}=\dfrac{10}{18}-\dfrac{3}{18}=\dfrac{10-3}{18}=\dfrac{7}{18}\)

c) \(\dfrac{8}{14}-\dfrac{2}{7}=\dfrac{8}{14}-\dfrac{4}{14}=\dfrac{8-4}{14}=\dfrac{4}{14}=\dfrac{2}{7}\)

d) \(\dfrac{12}{20}-\dfrac{2}{5}=\dfrac{12}{20}-\dfrac{8}{20}=\dfrac{12-8}{20}=\dfrac{4}{20}=\dfrac{1}{5}\)

\(\dfrac{14}{21}=\dfrac{7\cdot2}{7\cdot3}=\dfrac{2}{3}\)

\(\dfrac{-36}{48}=\dfrac{-12\cdot3}{12\cdot4}=-\dfrac{3}{4}\)

\(\dfrac{28}{-52}=\dfrac{7\cdot4}{-4\cdot13}=-\dfrac{7}{13}\)

\(\dfrac{-54}{-90}=\dfrac{-18\cdot3}{-18\cdot5}=\dfrac{3}{5}\)

\(\dfrac{14}{21}\) =\(\dfrac{2}{3}\)

\(\dfrac{-36}{48}\) =\(\dfrac{3}{4}\)

\(\dfrac{28}{-52}\)=\(\dfrac{7}{-13}\)

\(\dfrac{-54}{-90}\) =\(\dfrac{6}{9}=\dfrac{2}{3}\)

\(\dfrac{14}{21}=\dfrac{2}{3}\)

\(\dfrac{-36}{48}=-\dfrac{3}{4}\)

\(\dfrac{28}{-52}=-\dfrac{7}{13}\)

\(\dfrac{-54}{-90}=\dfrac{3}{5}\)

\(\dfrac{3}{12}+\dfrac{1}{4}=\dfrac{3:3}{12:3}+\dfrac{1}{4}=\dfrac{1}{4}+\dfrac{1}{4}=\dfrac{2}{4}=\dfrac{1}{2}\)

\(\dfrac{4}{10}+\dfrac{3}{5}=\dfrac{4:2}{10:2}+\dfrac{3}{5}=\dfrac{2}{5}+\dfrac{3}{5}=\dfrac{5}{5}=1\)

\(\dfrac{12}{27}+\dfrac{2}{9}=\dfrac{12:3}{27:3}+\dfrac{2}{9}=\dfrac{4}{9}+\dfrac{2}{9}=\dfrac{6}{9}=\dfrac{2}{3}\)

\(\dfrac{7}{3}+\dfrac{20}{15}=\dfrac{7}{3}+\dfrac{20:5}{15:5}=\dfrac{7}{3}+\dfrac{4}{3}=\dfrac{11}{3}\)

a) \(\dfrac{4}{24}+\dfrac{7}{6}=\dfrac{4}{24}+\dfrac{28}{24}=\dfrac{4+28}{24}=\dfrac{32}{24}=\dfrac{4}{3}\)

b) \(\dfrac{10}{15}-\dfrac{1}{3}=\dfrac{10}{15}-\dfrac{5}{15}=\dfrac{10-5}{15}=\dfrac{5}{15}=\dfrac{1}{3}\)

c) \(\dfrac{21}{28}-\dfrac{1}{4}=\dfrac{21}{28}-\dfrac{7}{28}=\dfrac{21-7}{28}=\dfrac{14}{28}=\dfrac{1}{2}\)

d) \(\dfrac{35}{40}+\dfrac{5}{8}=\dfrac{35}{40}+\dfrac{25}{40}=\dfrac{35+25}{40}=\dfrac{60}{40}=\dfrac{3}{2}\)

\(a)\dfrac{4}{24}=\dfrac{1}{6} \\ \dfrac{1}{6}+\dfrac{7}{6}\\ =\dfrac{8}{6}=\dfrac{4}{3}\\ b)\dfrac{10}{15}=\dfrac{2}{3}-\dfrac{1}{3}\\ =\dfrac{1}{3}\\ c)\dfrac{21}{28}=\dfrac{3}{4}\\ \dfrac{3}{4}-\dfrac{1}{4}\\ =\dfrac{2}{4}=\dfrac{1}{2}\\ d)\dfrac{35}{40}=\dfrac{7}{8}\\ \dfrac{7}{8}+\dfrac{5}{8}\\ =\dfrac{12}{8}=\dfrac{3}{2}\)