a: \(\dfrac{2}{3}+\dfrac{7}{3}=\dfrac{9}{3}\)

\(\dfrac{7}{3}+\dfrac{2}{3}=\dfrac{9}{3}\)

=>\(\dfrac{2}{3}+\dfrac{7}{3}=\dfrac{7}{3}+\dfrac{2}{3}\)

\(\dfrac{3}{5}+\dfrac{4}{5}=\dfrac{7}{5}\)

\(\dfrac{4}{5}+\dfrac{3}{5}=\dfrac{7}{5}\)

=>\(\dfrac{3}{5}+\dfrac{4}{5}=\dfrac{4}{5}+\dfrac{3}{5}\)

b: \(\dfrac{7}{9}+\dfrac{16}{9}=\dfrac{7+16}{9}=\dfrac{23}{9}\)

\(\dfrac{16}{9}+\dfrac{7}{9}=\dfrac{16+7}{9}=\dfrac{23}{9}\)

Do đó: \(\dfrac{7}{9}+\dfrac{16}{9}=\dfrac{16}{9}+\dfrac{7}{9}\)

a) \(\dfrac{2}{36}=\dfrac{1}{18}\)

\(\dfrac{8}{12}=\dfrac{2}{3}\)

b) \(\dfrac{10}{25}=\dfrac{2}{5}\)

\(\dfrac{14}{40}=\dfrac{7}{20}\)

\(\dfrac{-1}{2}=\dfrac{-9}{18};\dfrac{-5}{9}=\dfrac{-10}{18};\dfrac{-1}{3}=\dfrac{-6}{18}\)

mà -10<-9<-6<0

nên \(-\dfrac{5}{9}< -\dfrac{1}{2}< -\dfrac{1}{3}< 0\)(1)

Ta có: \(\dfrac{5}{12}=\dfrac{15}{36};\dfrac{7}{18}=\dfrac{14}{36};\dfrac{1}{3}=\dfrac{12}{36}\)

mà \(0< \dfrac{12}{36}< \dfrac{14}{36}< \dfrac{15}{36}\)

nên \(0< \dfrac{1}{3}< \dfrac{7}{18}< \dfrac{5}{12}\left(2\right)\)

Từ (1),(2) suy ra \(-\dfrac{5}{9}< -\dfrac{1}{2}< -\dfrac{1}{3}< \dfrac{1}{3}< \dfrac{7}{18}< \dfrac{5}{12}\)

1. A

2. C

3. C

\(\dfrac{1}{3}+\dfrac{2}{3}+\dfrac{4}{3}+\dfrac{19}{3}=\dfrac{1+2+4+19}{3}=\dfrac{26}{3}\)

\(\dfrac{3}{4}+\dfrac{4}{4}+\dfrac{5}{4}+\dfrac{6}{4}+\dfrac{x}{4}+\dfrac{8}{2}+\dfrac{9}{4}\)

=\(\dfrac{3}{4}+\dfrac{4}{4}+\dfrac{5}{4}+\dfrac{6}{4}+\dfrac{x}{4}+\dfrac{16}{4}+\dfrac{9}{4}\)

=\(\dfrac{3+4+5+6+x+16+9}{4}=\dfrac{43+x}{4}\)

Cảm ơn và chúc Lê Minh Quang học tốt nhé!

Mình đã tick rùi nha

Thanks

1/3+2/3+4/3+19/3=26/3
3/4+4/4+5/4+6/4+x/2+8/2+9/4=43+x/4

Bài 1:

a) 

\(\dfrac{1}{2}=\dfrac{1\times6}{2\times6}=\dfrac{6}{12}\)

\(\dfrac{2}{3}=\dfrac{2\times4}{3\times4}=\dfrac{8}{12}\)

\(\dfrac{3}{4}=\dfrac{3\times3}{4\times3}=\dfrac{9}{12}\)

b)

\(\dfrac{1}{3}=\dfrac{1\times15}{3\times15}=\dfrac{15}{45}\)

\(\dfrac{2}{15}=\dfrac{2\times3}{15\times3}=\dfrac{6}{45}\)

\(\dfrac{4}{45}\) (giữ nguyên)

c)

\(\dfrac{1}{8}=\dfrac{1\times3}{8\times3}=\dfrac{3}{24}\)

\(\dfrac{2}{3}=\dfrac{2\times8}{3\times8}=\dfrac{16}{24}\)

\(\dfrac{5}{2}=\dfrac{5\times12}{2\times12}=\dfrac{60}{24}\)

d)

\(\dfrac{2}{7}=\dfrac{2\times4}{7\times4}=\dfrac{8}{28}\)

\(\dfrac{9}{4}=\dfrac{9\times7}{4\times7}=\dfrac{63}{28}\)

\(\dfrac{5}{28}\) (giữ nguyên)

Bài 2:

a)

\(4=\dfrac{4}{1}=\dfrac{4\times12}{1\times12}=\dfrac{48}{12}\)

\(\dfrac{9}{4}=\dfrac{9\times3}{4\times3}=\dfrac{27}{12}\)

b)

\(\dfrac{5}{8}=\dfrac{5\times30}{8\times30}=\dfrac{150}{240}\)

\(\dfrac{25}{30}=\dfrac{5}{6}=\dfrac{5\times40}{6\times40}=\dfrac{200}{240}\)

\(2=\dfrac{2}{1}=\dfrac{2\times240}{1\times240}=\dfrac{480}{240}\).

giúp mình với ạ!

a) \(\dfrac{3}{4}=\dfrac{3\times4}{4\times4}=\dfrac{12}{16}\)

b) \(\dfrac{1}{3}=\dfrac{1\times3}{3\times3}=\dfrac{3}{9}\)

c) \(\dfrac{5}{6}=\dfrac{5\times3}{6\times3}=\dfrac{15}{18}\)

a) \(\dfrac{1}{6}=\dfrac{3}{18}\)

b) \(\dfrac{4}{5}=\dfrac{48}{60}\)

c) \(\dfrac{7}{25}=\dfrac{28}{100}\)

a: 1/2 của 18 là \(\dfrac{1}{2}\text{x}18=\dfrac{18}{2}=9\)

1/7 của42 là \(\dfrac{1}{7}\text{x}42=\dfrac{42}{7}=6\)

b: 3/4 của 20 là \(\dfrac{3}{4}\text{x}20=\dfrac{3\text{x}20}{4}=\dfrac{60}{4}=15\)

3/7 của 21 là \(\dfrac{3}{7}\text{x}21=\dfrac{3\text{x}21}{7}=\dfrac{63}{7}=9\)

c:2/5 của 35 là: \(35\text{x}\dfrac{2}{5}=\dfrac{35\text{x}2}{5}=14\)

2/9 của 36 là \(\dfrac{2}{9}\text{x}36=\dfrac{2\text{x}36}{9}=\dfrac{72}{9}=8\)