1. -2x=5y =>\(\frac{x}{y}=\frac{-5}{2}=>y=\frac{-2x}{5}\)

Thế y=\(\frac{-2x}{5}\) ta được:

x+\(\frac{-2x}{5}\)=30     \(\Rightarrow\frac{5x-2x}{5}=30\)

\(\Rightarrow3x=150\)\(\Rightarrow x=50\)

=>y=30-x=30-50=-20.

Vậy x=50; y=-20.

Những bài khác tương tự bạn nhé!

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thank

Áp dụng t/c dãy tỉ số bằng nhau:

a.

\(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{x+y}{2+5}=\dfrac{-21}{7}=-3\)

\(\Rightarrow\left\{{}\begin{matrix}x=2.\left(-3\right)=-6\\y=5.\left(-3\right)=-15\end{matrix}\right.\)

b.

\(5x=3y\Rightarrow\dfrac{x}{3}=\dfrac{y}{5}=\dfrac{x-y}{3-5}=\dfrac{10}{-2}=-5\)

\(\Rightarrow\left\{{}\begin{matrix}x=3.\left(-5\right)=-15\\y=5.\left(-5\right)=-25\end{matrix}\right.\)

c.

\(\dfrac{x}{5}=\dfrac{y}{2}=\dfrac{3x}{15}=\dfrac{-2y}{-4}=\dfrac{3x-2y}{15-4}=\dfrac{44}{11}=4\)

\(\Rightarrow\left\{{}\begin{matrix}x=5.4=20\\y=2.4=8\end{matrix}\right.\)

d.

\(\dfrac{x}{3}=\dfrac{y}{16}=\dfrac{3x}{9}=\dfrac{-y}{-16}=\dfrac{3x-y}{9-16}=\dfrac{35}{-7}=-5\)

\(\Rightarrow\left\{{}\begin{matrix}x=3.\left(-5\right)=-15\\y=16.\left(-5\right)=-80\end{matrix}\right.\)

\(a,3x=2y\)và \(x+y=10\)

Ta cs : \(3x=2y\Rightarrow\frac{x}{2}=\frac{y}{3}\)

ADTC dãy tỉ số bằng nhau ta cs

\(\frac{x}{2}=\frac{y}{3}=\frac{x+y}{2+3}=\frac{10}{5}=2\)

\(\Leftrightarrow\frac{x}{2}=2\Leftrightarrow x=4\)

\(\Leftrightarrow\frac{y}{3}=2\Leftrightarrow y=6\)

\(c,\frac{x}{2}=\frac{y}{5}\)và \(x+2y=12\)

ADTC dãy tỉ số bằng nhau ta cs

\(\frac{x}{2}=\frac{y}{5}=\frac{x+2y}{2+2.5}=\frac{12}{12}=1\)

\(\Leftrightarrow\frac{x}{2}=1\Leftrightarrow x=2\)

\(\Leftrightarrow\frac{y}{5}=1\Leftrightarrow y=5\)

bạn k lm phần b hộ mình ak 

ko ADTC dãy tỉ số = nhau đâu :(( 

\(b,\frac{x-2}{y+3}=\frac{8}{2}\)và \(y-x=-4\)

\(\frac{x-2}{8}=\frac{y+3}{2}\Leftrightarrow\frac{x-2}{8}=\frac{4y+12}{8}\)

\(\Leftrightarrow x-2=4y+12\)

\(\Leftrightarrow x-4y=12+2=14\Leftrightarrow x-4y=14\)

\(\Leftrightarrow x=14+4y\Leftrightarrow x=\frac{14y}{y}+\frac{4y}{1}\Leftrightarrow\frac{14y}{y}+\frac{4y}{y}=\frac{18y}{y}=18\)

Vậy x = 18 

Thay x = 18 

\(\frac{18-2}{y+3}=\frac{8}{2}\Leftrightarrow\frac{16}{y+3}=4\Leftrightarrow y+3=4\Leftrightarrow y=1\)

Vậy y = 1 

a)

\(\dfrac{x}{5}=\dfrac{y}{2}=\dfrac{3x-2y}{3.5-2.2}=\dfrac{-55}{11}=-5\)

=> \(\left\{{}\begin{matrix}x=-5.5=-25\\y=-5.2=-10\end{matrix}\right.\)

b)

\(\dfrac{x}{3}=\dfrac{y}{2}=\dfrac{2x+5y}{2.3+5.2}=\dfrac{48}{16}=3\)

=> \(\left\{{}\begin{matrix}x=3.3=9\\y=3.2=6\end{matrix}\right.\)

c)

Có: \(\dfrac{x}{y}=-\dfrac{5}{2}\Leftrightarrow-\dfrac{x}{5}=\dfrac{y}{2}=\dfrac{x+y}{-5+2}=\dfrac{30}{-3}=-10\)

=> \(\left\{{}\begin{matrix}x=-10.-5=50\\y=-10.2=-20\end{matrix}\right.\)

d)

Có: \(\dfrac{x}{y}=\dfrac{4}{3}\Leftrightarrow\dfrac{x}{4}=\dfrac{y}{3}=\dfrac{2x+3y}{2.4+3.3}=\dfrac{34}{17}=2\)

=> \(\left\{{}\begin{matrix}x=2.4=8\\y=2.3=6\end{matrix}\right.\)

x/2=y/3;y/2=z/5 => x/2=2y/6;3y/6=z/5 => x/4=y/6=z/15

adtcdtsbn:

x/4=y/6=z/15=x+y+z/4+6+15=50/25=2

suy ra : x/4=2=>x=4.2=8

y/6=2=>y=2.6=12

z/15=2 => z=15.2=30

`-3x=2y `

`=> x/2 = -y/3 `

AD t/c của dãy tỉ số bằng nhau ta có

`x/2 =-y/3 = (x-y)/(2+3) = 6/5`

`=>{(x=2*6/5 = 12/5),(y=-3*6/5 =-18/5):}`

a) `6/x =-3/2`

`=>x =6 :(-3/2) = 6*(-2/3)=-4`

`b)`\(-3x=2y\Rightarrow\dfrac{x}{2}=\dfrac{y}{-3}\)

Áp dụng t/c của DTSBN , ta đc :

\(\dfrac{x}{2}=\dfrac{y}{-3}=\dfrac{x-y}{2+3}=\dfrac{6}{5}\\ \Rightarrow\left\{{}\begin{matrix}\dfrac{x}{2}=\dfrac{6}{5}\\\dfrac{y}{-3}=\dfrac{6}{5}\end{matrix}\right.\Rightarrow\left\{{}\begin{matrix}x=\dfrac{12}{5}\\y=-\dfrac{18}{5}\end{matrix}\right. \)

`a)`

`6/x=-3/2`

`x=6:(-3/2)`

`x=6*(-2/3)`

`x=-4`

a, \(\dfrac{6}{x}=-\dfrac{3}{2}\)

\(\Rightarrow3x=6.\left(-2\right)\)

\(\Rightarrow3x=-12\)

\(\Rightarrow x=-\dfrac{12}{3}\)

=>x=-4

b, \(+\left(-3\right)x=2y\Rightarrow\dfrac{x}{2}=\dfrac{y}{-3}\)

+x-y=6

+Áp dụng tính chất dãy tỉ số bằng nhau có:

 \(\dfrac{x}{2}=\dfrac{y}{-3}=\dfrac{x-y}{2-\left(-3\right)}=\dfrac{6}{5}\)

Suy ra \(\dfrac{x}{2}=\dfrac{6}{5}\Rightarrow x=\dfrac{6}{5}.2\Rightarrow x=\dfrac{12}{5}\)

           \(\dfrac{y}{-3}=\dfrac{6}{5}\Rightarrow y=\dfrac{6}{5}.\left(-3\right)=-\dfrac{18}{5}\)

Vậy \(x=\dfrac{12}{5};y=-\dfrac{18}{5}\)

Câu hỏi của Trang Đinh Huyền - Toán lớp 7 - Học toán với OnlineMath