a) Ta có: 3.4 = 2.6
\(\Rightarrow\frac{3}{2}=\frac{6}{4};\)\(\frac{3}{6}=\frac{2}{4};\)\(\frac{4}{2}=\frac{6}{3};\)\(\frac{4}{6}=\frac{2}{3}\)
b) Ta có: 14.15 = 10.21
\(\Rightarrow\frac{14}{10}=\frac{21}{15};\)\(\frac{14}{21}=\frac{10}{15};\)\(\frac{15}{10}=\frac{21}{14};\)\(\frac{15}{21}=\frac{10}{14}\)
c) Ta có: AB.CD = 2.3
\(\Rightarrow\frac{AB}{2}=\frac{3}{CD};\)\(\frac{AB}{3}=\frac{2}{CD};\)\(\frac{CD}{2}=\frac{3}{AB};\)\(\frac{CD}{3}=\frac{2}{AB}\)
d) Ta có: A.AB = 5.MN
\(\Rightarrow\frac{A}{5}=\frac{MN}{AB};\)\(\frac{A}{MN}=\frac{5}{AB};\)\(\frac{AB}{5}=\frac{MN}{A};\)\(\frac{AB}{MN}=\frac{5}{A}\)

a)3.4=2.6

=\(\frac{3}{2}=\frac{6}{4};\frac{3}{6}=\frac{2}{4};\frac{2}{3}=\frac{4}{6};\frac{6}{3}=\frac{4}{2}\)

b)14.15=10.21

=\(\frac{14}{10}=\frac{21}{15};\frac{14}{21}=\frac{10}{15};\frac{15}{10}=\frac{21}{14};\frac{15}{21}=\frac{10}{14}\)

c)AB.CD=2.3

=\(\frac{AB}{2}=\frac{3}{CD};\frac{AB}{3}=\frac{2}{CD};\frac{CD}{2}=\frac{3}{AB};\frac{CD}{3}=\frac{2}{AB}\)

D)A.AB=5.MN

=\(\frac{A}{5}=\frac{MN}{AB};\frac{A}{MN}=\frac{5}{AB};\frac{AB}{5}=\frac{MN}{A};\frac{AB}{MN}=\frac{5}{A}\)

\(\frac{2}{3}=\frac{4}{6}\left(1\right)\)

\(\frac{2}{4}=\frac{3}{6}\left(2\right)\)

\(\frac{3}{2}=\frac{6}{4}\left(3\right)\)

\(\frac{4}{2}=\frac{6}{3}\left(4\right)\)

 Ticks  nha

2.6=3.4

=>2/3=4/6

2/4=3/6

6/4=3/2

6/3=4/2

a) \(\frac{a}{3b}\)=  \(\frac{c}{3d}\)

Đặt \(\frac{a}{b}\)= \(\frac{c}{d}\)= k

=.> a = b.k, c = d.k

Từ đó ta có 

+) \(\frac{a}{c-3b}\)= \(\frac{bk}{3k-3b}\)= \(\frac{bk}{b.\left(k-3\right)}\)= \(\frac{k}{k-3}\)                          (1)

+) \(\frac{c}{c-3d}\)= \(\frac{dk}{dk-3d}\)-= \(\frac{dk}{d.\left(k-3\right)}\)= \(\frac{k}{k-3}\)                         (2)

Từ (1) và (2) ta có

\(\frac{a}{a-3b}\) = \(\frac{c}{c-3d}\)

Bài 1:

a: \(3\cdot4=2\cdot6\)

=>\(\frac32=\frac64;\frac36=\frac24;\frac23=\frac46;\frac63=\frac42\)

b: \(4\cdot AB=5\cdot MN\)

=>\(\frac{AB}{5}=\frac{MN}{4};\frac{AB}{MN}=\frac54;\frac{5}{AB}=\frac{4}{MN};\frac{MN}{AB}=\frac45\)

Bài 2:

a: \(\frac{2.6}{x}=\frac{-12}{42}\)

=>\(\frac{2.6}{x}=\frac{-2}{7}\)

=>\(x=2,6\cdot\frac{7}{-2}=-1,3\cdot7=-9,1\)

b: \(\frac{x+3}{-4}=\frac{5}{20}\)

=>\(\frac{x+3}{-4}=\frac14\)

=>x+3=-1

=>x=-4

c: \(\frac{17-7x}{-4}=\frac{4-3x}{-5}\)

=>\(\frac{7x-17}{4}=\frac{3x-4}{5}\)

=>5(7x-17)=4(3x-4)

=>35x-85=12x-16

=>35x-12x=-16+85

=>23x=69

=>x=3

Các tỉ lệ thức lập được từ 15.12 = 9.20 là:  \(\frac{15}{9}\)=  \(\frac{20}{12}\);    \(\frac{15}{20}\)=  \(\frac{9}{12}\);  \(\frac{12}{20}\)= \(\frac{9}{15}\);   \(\frac{20}{15}\)= \(\frac{12}{9}\)

Mình cũng lớp 7: mình làm như sau:
15/9=20/12;15/20=9/12;12/9=20/15;12/20=9/15
ta lập đc 4 tỉ lệ thức
k mình nha