Ta có:

\(\dfrac{3x-y}{x+y}=\dfrac{3}{4}\\ \Rightarrow12x-4y=3x+3y\\ \Rightarrow9x=7y\\ \Rightarrow\dfrac{x}{y}=\dfrac{7}{9}\)

Vậy.........

\(\dfrac{3x-y}{x+y}=\dfrac{3}{4}\Rightarrow\left(3x-y\right).4=3\left(x+y\right)\)

\(\Rightarrow12x-4y=3x+3y\)

\(\Rightarrow9x=7y\)

\(\Rightarrow\)\(\dfrac{x}{y}=\dfrac{7}{9}\)

Lời giải:

 $\frac{x}{y}=\frac{2}{3}\Rightarrow \frac{x}{2}=\frac{y}{3}$. Đặt $\frac{x}{2}=\frac{y}{3}=k$ thì:

$x=2k; y=3k$

Khi đó: $3x-2y=3.2k-3.2k=0$. Mẫu số không thể bằng $0$ nên $A$ không xác định. Bạn xem lại.

$B=\frac{2(2k)^2-2k.3k+3(3k)^2}{3(2k)^2+2.2k.3k+(3k)^2}=\frac{29k^2}{33k^2}=\frac{29}{33}$

\(\dfrac{3x-y}{x+y}=\dfrac{3}{4}\Rightarrow4\left(2x-y\right)=3\left(x+y\right)\)

\(\Rightarrow12x-4y=3x+3y\Rightarrow12x-3x=3y+4y\)

\(\Rightarrow9x=7y\Rightarrow\dfrac{x}{y}=\dfrac{7}{9}\)

sửa lại cho mik dòng đầu là 4(3x-y)

\(\dfrac{3x-y}{x+y}=\dfrac{3}{4}\)

\(\Leftrightarrow4\left(3x-y\right)=3\left(x+y\right)\)

\(\Leftrightarrow12x-4y=3x+3y\)

\(\Leftrightarrow12x=3x+7y\)

\(\Leftrightarrow9x=7y\)

\(\Leftrightarrow\dfrac{x}{7}=\dfrac{y}{9}\Leftrightarrow\dfrac{x}{y}=\dfrac{7}{9}\)

Ta có : \(\dfrac{3x-y}{x+y}=\dfrac{3}{4}\)

\(\Leftrightarrow4\left(3x-y\right)=3\left(x+y\right)\)

\(\Leftrightarrow12x-4y=3x+3y\Rightarrow15x=7y\)

\(\Rightarrow\dfrac{x}{y}=\dfrac{7}{15}\)

tik mik nha !!!

\(\dfrac{3x-y}{x+y}=\dfrac{3}{4}\)

=> 4(3x-y)=3(x+y)

=> 12x-4y=3x+3y

=> 12x-3x=4y+3y

=> 9x=7y

=> \(\dfrac{x}{y}=\dfrac{7}{9}\)

\(\dfrac{3x-y}{x+y}=\dfrac{3}{4}\\ \Rightarrow4\cdot\left(3x-y\right)=3\cdot\left(x+y\right)\\ \Leftrightarrow12x-4y=3x+3y\\ \Leftrightarrow12x-3x=3y+4y\\ \Leftrightarrow9x=7y\\ \Rightarrow\dfrac{x}{7}=\dfrac{y}{9}\\ \Rightarrow\dfrac{x}{y}=\dfrac{7}{9}\)

Vậy \(\dfrac{x}{y}=\dfrac{7}{9}\)

Ta có: \(\dfrac{3x-y}{x+y}=\dfrac{3}{4}\)

\(\Rightarrow\left(3x-y\right)4=\left(x+y\right)3\)

\(\Rightarrow12x-4y=3x+3y\)

\(\Rightarrow12x-3x=4y+3y\)

\(\Rightarrow9x=7y\)

\(\Rightarrow\dfrac{x}{y}=\dfrac{7}{9}\)

Vậy \(\dfrac{x}{y}=\dfrac{7}{9}.\)

\(\dfrac{3x-y}{x+y}=\dfrac{3}{4}\)

\(\Rightarrow4\left(3x-y\right)=3\left(x+y\right)\)

\(\Rightarrow12x-4y=3x+3y\)

\(\Rightarrow12x-4y-3y=3x\)

\(\Rightarrow12x-7y=3x\)

\(\Rightarrow9x=7y\)

\(\Rightarrow x=\dfrac{7}{9}y\)

\(\Rightarrow\dfrac{x}{y}=\dfrac{7}{9}\)

Bài 1: 

Ta có: \(3x=2y\)

nên \(\dfrac{x}{2}=\dfrac{y}{3}\)

mà x+y=-15

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

\(\dfrac{x}{2}=\dfrac{y}{3}=\dfrac{x+y}{2+3}=\dfrac{-15}{5}=-3\)

Do đó:

\(\left\{{}\begin{matrix}\dfrac{x}{2}=-3\\\dfrac{y}{3}=-3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=-6\\y=-9\end{matrix}\right.\)

Vậy: (x,y)=(-6;-9)

Bài 2: 

a) Ta có: \(\dfrac{x}{4}=\dfrac{y}{3}=\dfrac{z}{5}\)

mà x+y-z=20

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

\(\dfrac{x}{4}=\dfrac{y}{3}=\dfrac{z}{5}=\dfrac{x+y-z}{4+3-5}=\dfrac{20}{2}=10\)

Do đó:

\(\left\{{}\begin{matrix}\dfrac{x}{4}=10\\\dfrac{y}{3}=10\\\dfrac{z}{5}=10\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=40\\y=30\\z=50\end{matrix}\right.\)

Vậy: (x,y,z)=(40;30;50)

Bài 2: 

b) Ta có: \(\dfrac{y}{3}=\dfrac{z}{7}\)

nên \(\dfrac{y}{12}=\dfrac{z}{28}\)

mà \(\dfrac{x}{11}=\dfrac{y}{12}\)

nên \(\dfrac{x}{11}=\dfrac{y}{12}=\dfrac{z}{28}\)

hay \(\dfrac{2x}{22}=\dfrac{y}{12}=\dfrac{z}{28}\)

mà 2x-y+z=152

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

\(\dfrac{2x}{22}=\dfrac{y}{12}=\dfrac{z}{28}=\dfrac{2x-y+z}{22-12+28}=\dfrac{152}{38}=4\)

Do đó:

\(\left\{{}\begin{matrix}\dfrac{x}{11}=4\\\dfrac{y}{12}=4\\\dfrac{z}{28}=4\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=44\\y=48\\z=112\end{matrix}\right.\)

Vậy: (x,y,z)=(44;48;112)

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