Áp dụng tính chất của dãy tỉ số bằng nhau, ta có:

\(\begin{array}{l}\frac{x}{5} = \frac{y}{7} = \frac{z}{9} = \frac{{x - y + z}}{{5 - 7 + 9}} = \frac{{\frac{7}{3}}}{7} = \frac{7}{3}.\frac{1}{7} = \frac{1}{3}\\ \Rightarrow x = 5.\frac{1}{3} = \frac{5}{3};\\y = 7.\frac{1}{3} = \frac{7}{3};\\z = 9.\frac{1}{3} = \frac{9}{3} = 3.\end{array}\)

Vậy \(x = \frac{5}{3};y = \frac{7}{3};z = 3\)

(\(\frac{5}{7}-y\)) x \(\frac{14}{5}=\frac{6}{5}\)

\(\frac{5}{7}-y\)            = \(\frac{6}{5}:\frac{14}{5}\)

\(\frac{5}{7}-y\)             = \(\frac{3}{7}\)

            y                 = \(\frac{5}{7}-\frac{3}{7}\)

           y                  = \(\frac{2}{7}\)

\(\left(\frac{5}{7}-y\right)\cdot\frac{14}{5}=\frac{7}{10}+\frac{1}{2}\)

\(\Leftrightarrow\left(\frac{5}{7}-y\right)\cdot\frac{14}{5}=\frac{7}{10}+\frac{5}{10}\)

\(\Leftrightarrow\left(\frac{5}{7}-y\right)\cdot\frac{14}{5}=\frac{6}{5}\)

\(\Leftrightarrow\frac{5}{7}-y=\frac{6}{5}\div\frac{14}{5}\)

\(\Leftrightarrow\frac{5}{7}-y=\frac{3}{7}\)

\(\Rightarrow y=\frac{5}{7}-\frac{3}{7}=\frac{2}{7}\)

Nên tự tính 

Nếu thik thì lấy máy tính tính cho nhanh nha bạn

1, ta co \(\frac{x}{5}=\frac{y}{6}=\frac{x}{20}=\frac{y}{24}\)

\(\frac{y}{8}=\frac{z}{7}=\frac{y}{24}=\frac{z}{21}\)

=>\(\frac{x}{20}=\frac{y}{24}=\frac{z}{21}=\frac{x+y-z}{20+24-21}=\frac{69}{23}=3\)

=>\(x=3\cdot20=60\)

    \(y=3\cdot24=72\)

    \(z=3\cdot21=63\)

3. ta co \(\frac{x}{15}=\frac{y}{7}=\frac{z}{3}=\frac{t}{1}=\frac{x+y-z+t}{15-7+3-1}=\frac{10}{10}=1\)

=> \(x=1\cdot15=15\)

     \(y=1\cdot7=7\)

     \(z=1\cdot3=3\)

     \(t=1\cdot1=1\)

Áp dụng tính chất dãy tỉ số bằng nhau

\(\frac{x}{5}=\frac{y}{7}=\frac{z}{9}=\frac{x-y+z}{5-7+9}=\frac{315}{7}=45\)

  suy ra:   x/5 = 45   =>  x  =  225

               y/7 = 45  =>  y  =  315

               z/9 = 45  =>  z  =  405

(x-2)/5=(y+5)/7=(x-2+y+5)/12=(x+y-2+5)/12

=(21-2+5)/12=2

=>(x-2)/5=2=>x=12

=>(y+5)/7=2=>y=9

a)ta có xy=7*9=7*3*3

vậy x =9;21 , y=7;3

b) xy=-2*5

mà x<0<y

nên x=-2 ,y=5

c)x-y=5 hay x=y+5

\(\frac{y+5+4}{y-5}=\frac{4}{3}\Rightarrow3y+27=4y-20\Rightarrow y=47\Rightarrow x=52\)

câu c mk nhầm đề sr bạn nha

\(\frac{y+5-4}{y-5}=\frac{4}{3}\Rightarrow3y+3=4y-5\Rightarrow y=8\Rightarrow x=13\)

\(\frac{x+3}{y+5}=\frac{x+5}{y+7}=\frac{x+5-x-3}{y+7-y-5}=\frac{2}{2}=1\)

=> x+3 = y+5

=> x -y = 5 -3 =2

Vậy x -y =2

\(\Rightarrow\left(x+3\right)\left(y+7\right)=\left(x+5\right)\left(y+5\right)\)

\(\Rightarrow xy+7x+3y+21=xy+5x+5y+25\)

\(\Rightarrow2x-2y=4\Rightarrow x-y=2\)

\(\frac{x+3}{y+5}=\frac{x+5}{y+7}\)

=>(x+3).(y+7)=(x+5).(y+5)

=>x.y+7x+3y+21=xy+5x+5y+25

=>2x-2y=4

=>2(x-y)=4

=>x-y=4:2

=>x-y=2