a) \(\dfrac{15}{13}>\dfrac{13}{13}=>\dfrac{15}{13}>1\\ \dfrac{35}{37}< \dfrac{37}{37}=>\dfrac{35}{37}< 1\\ =>\dfrac{15}{13}>\dfrac{35}{37}\)

b)\(\dfrac{19}{20}< \dfrac{20}{20}=>\dfrac{19}{20}< 1\\ \dfrac{39}{38}>\dfrac{38}{38}=>\dfrac{39}{38}>1\\ =>\dfrac{19}{20}< \dfrac{39}{38}\)

Gọi số phải tìm là : \(\overline{ab}\)

Số mới là : \(\overline{nabn}\)

Ta có :

\(\overline{ab}\times21=\overline{nabn}\\ \overline{ab}\times21=\overline{n00n}+\overline{ab0}\\ \overline{ab}\times21=n\times1000+n+\overline{ab}\times10\\ \overline{ab}\times11=n\times1001\\ \overline{ab}=n\times91\\ =>n=1;\overline{ab}=91\)

minh ở đấu

1234231

5/6; 51/42; 14/21 ;15/14 ;8/7

= 35/42 ; 51/42 ; 28/42 ; 45 /42 ; 48/42

= 28/42; 35/42;45/42 ;  48/42;51/42

=> 14/21 ; 5/6 ; 15/14 ; 8/7 ; 51/42

\(a)\dfrac{3}{4} \times \dfrac{a}{b}-\dfrac{5}{6}=\dfrac{1}{3}\)

\(\dfrac{3}{4} \times \dfrac{a}{b}=\dfrac{1}{3}+\dfrac{5}{6}\)

\(\dfrac{3}{4} \times \dfrac{a}{b}=\dfrac{2}{6}+\dfrac{5}{6}=\dfrac{7}{6}\)

\(\dfrac{a}{b}=\dfrac{7}{6}:\dfrac{3}{4}\)

\(\dfrac{a}{b}=\dfrac{7}{6} \times \dfrac{4}{3}=\dfrac{14}{9}\)

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\(b)\dfrac{10}{7}-\dfrac{7}{9}:\dfrac{a}{b}=\dfrac{13}{14}\)

\(\dfrac{7}{9}:\dfrac{a}{b}=\dfrac{10}{7}-\dfrac{13}{14}\)

\(\dfrac{7}{9}:\dfrac{a}{b}=\dfrac{20}{14}-\dfrac{13}{14}=\dfrac{7}{14}\)

\(\dfrac{a}{b}=\dfrac{7}{9}:\dfrac{7}{14}\)

\(\dfrac{a}{b}=\dfrac{7}{9} \times \dfrac{14}{7}=\dfrac{14}{9}\)

giúp mk với 

`MSC:585`

\(\dfrac{2}{5}=\dfrac{2 \times 117}{5 \times 117}=\dfrac{234}{585}\)

\(\dfrac{7}{9}=\dfrac{7 \times 65}{9 \times 65}=\dfrac{455}{585}\)

\(\dfrac{11}{13}=\dfrac{11 \times 45}{13 \times 45}=\dfrac{495}{585}\)