a: =>4/x=y/21=4/7

=>x=7; y=21*4/7=12

b: x/7=9/y

=>xy=63

mà x>y

nên \(\left(x,y\right)\in\left\{\left(63;1\right);\left(21;3\right);\left(9;7\right);\left(-7;-9\right);\left(-3;-21\right);\left(-1;-63\right)\right\}\)

c: x/15=3/y

=>xy=45

mà x<y<0

nên \(\left(x,y\right)\in\left\{\left(-45;-1\right);\left(-15;-3\right);\left(-9;-5\right)\right\}\)

d: x/y=21/28=3/4

=>x/3=y/4=k

=>x=3k; y=4k(k\(\in Z\))

Lời giải:

a. $\frac{x}{7}=\frac{6}{21}$

$x=\frac{6}{21}.7$

$x=2$

b.

$\frac{-5}{y}=\frac{20}{28}$

$y=-5:\frac{20}{28}$

$y=-7$

c.

$\frac{-4}{8}=\frac{-7}{y}$

$y=-7:\frac{-4}{8}$

$y=14$

a, \(\dfrac{x}{7}=\dfrac{6}{21}\Leftrightarrow\dfrac{3x}{21}=\dfrac{6}{21}\Rightarrow x=2\)

b, \(\dfrac{-5}{y}=\dfrac{20}{28}\Leftrightarrow\dfrac{20}{-4y}=\dfrac{20}{28}\Leftrightarrow y=-7\)

c, \(\dfrac{-4}{8}=-\dfrac{7}{y}\Rightarrow-4y=-56\Leftrightarrow y=14\)

a) Ta có: \(\dfrac{x}{7}=\dfrac{6}{21}\)

nên \(x=\dfrac{6\cdot7}{21}=\dfrac{42}{21}=2\)

b) Ta có: \(\dfrac{-5}{y}=\dfrac{20}{28}\)

nên \(y=\dfrac{-5\cdot28}{20}=\dfrac{-140}{20}=-7\)

c) Ta có: \(\dfrac{-4}{8}=\dfrac{-7}{y}\)

nên \(y=\dfrac{-7\cdot8}{-4}=\dfrac{-56}{-4}=14\)

?????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????????//

a) \(\frac{x}{6}=\frac{5}{24}\Rightarrow x=\frac{5}{24}.6=\frac{5}{4}\)

b) \(-\frac{4}{y}=\frac{20}{14}\Rightarrow y=-4:\frac{20}{14}=-\frac{14}{5}\)

c) \(\frac{4}{7}=\frac{12}{x}\Rightarrow x=12:\frac{4}{7}=21\)

d) \(\frac{3}{7}=\frac{y}{21}\Rightarrow y=\frac{3}{7}.21=9\)

a.x/3=7/y

suy ra 3.7=x.y

suy ra 21=x.y

mà 21=-3.-7=3.7=1.21=-1.-21

vậy (x;y)E{(-3;-7);(-7;-3);(7;3);(3;7);(1;21);(21;1);(-1;-21);(-21;-1)}

b.x-3/27=7/21

suy ra (x-3).21=27.7

suy ra (x-3).21=189

suy ra(x-3)=189:21=9

suy ra x=9+3=12

vậy x =12

k cho mình như

\(\frac{x}{6}=\frac{7}{y}\)

<=> xy=7.6=42

x , y nguyên và x<y<0 nên

\(\frac{x}{6}=\frac{7}{y}\)

APTC dãy j j bằng nhau ( đc ko , tui cx chả nhớ cái đó nữa )

\(\Leftrightarrow xy=7.6\)

Do đó : \(\frac{x}{6}=42\)hay \(x:6=42\)

\(\Leftrightarrow x=42.6\Leftrightarrow x=252\)

\(\frac{7}{y}=42\)hay \(7:y=42\)

\(\Leftrightarrow y=7:42\Leftrightarrow y=\frac{7}{42}\)

Vì theo đề bài ta có x<y<0 

=> \(\Rightarrow\hept{\begin{cases}x=252\\y=\frac{7}{42}\left(l\right)\end{cases}}\)toi cảm thấy sai sai hay sao ý 

\(\frac{x}{12}=\frac{5}{4}\)l

\(\Rightarrow x=\frac{5.12}{4}\)

\(\Rightarrow x=15\)

\(\frac{-8}{x}=\frac{16}{6}\)

\(\Rightarrow x=\frac{-8.6}{16}\)

\(\Rightarrow x=-3\)

làm giúp mình 2 bài cuối nha bạn tks trước

\(\frac{-7}{21}=\frac{x}{9}\)

\(\Rightarrow x=\frac{-7.9}{21}\)

\(\Rightarrow x=-3\)

\(\frac{-x}{15}=\frac{7}{3}\)

\(\Rightarrow\frac{x}{-15}=\frac{7}{3}\)

\(\Rightarrow x=\frac{-15.7}{3}\)

\(\Rightarrow x=-35\)