x=-9, y=-49/3

Ta có 

7/x = -42/54 => x= 7.54/-42 =>x=-9 (tmđk)

y/21 = -42/54 => y= -42.21/54 => y= -49/3 

\(\frac{7}{x}=\frac{y}{21}=\frac{-42}{54}\)

Ta có : \(\frac{-42}{54}=\frac{-42:6}{54:6}=\frac{-7}{9}\)

+) \(\frac{7}{x}=\frac{-7}{9}\)=> 7.9 = (-7).x => (-7).x = 63 => x = -9

+) \(\frac{y}{21}=\frac{-7}{9}\)=> y.9 = 21.(-7) => y.9 = -147 => y = -49/3

Vậy x = 9,y = -49/3

\(\dfrac{4}{x}=\dfrac{y}{21}=\dfrac{28}{49}=\dfrac{28:7}{49:7}=\dfrac{4}{9}\\ Vậy:x=\dfrac{4.9}{4}=9\\ y=\dfrac{4.21}{9}=\dfrac{28}{3}\)

\(\dfrac{x}{2}=\dfrac{3}{y}\\ \Leftrightarrow x.y=2.3=6\\ Vậy:\left[{}\begin{matrix}\left(x;y\right)=\left(1;6\right)=\left(6;1\right)\\\left(x;y\right)=\left(2;3\right)=\left(3;2\right)\end{matrix}\right.\)

x=24

x=3

x=9

x=75

\(\frac{3}{7}=\frac{x}{56}=\frac{24}{56}=>x=24\)

\(\frac{x}{5}=\frac{21}{35}=\frac{x}{5}=\frac{3}{5}=>x=3\)

\(\frac{42}{54}=\frac{7}{x}=\frac{7}{8}=\frac{7}{x}=>x=8\)

\(\frac{35}{x}=\frac{7}{15}=\frac{35}{x}=\frac{35}{75}=>x=75\)

1) 42/35 - [ 33/14 . 21/22 - ( 54/25 : 27/25 - 7/10 )]

= 42/35 - ( 9/4-13/10)

=42/35-19/20

=1/4

2) a) 5/21 . x/34 = 35/102     

x/34=35/102 : 5/21

x/34=49/34

=> x=34

Vậy x=34

b) 15/x . 21/13 = 45/91

15/x=45/91 : 21/13

15/x=15/49

=> x=49

Vậy x=49

lớp 4

\(\frac{7}{x}=\frac{y}{81}=-\frac{42}{54}\)

\(+,\frac{7}{x}=-\frac{42}{54}\Rightarrow378=-42x\Rightarrow x=-9\)

\(+,\frac{y}{81}=-\frac{42}{54}\Rightarrow54y=-3402\Rightarrow y=-63\)

a, x + 26 = 39 + (-5)

   x + 26 = 39 - 5

   x + 26 = 34

   x        = 34 - 26

   x        = 8

b, |x| + 50 = (-|-26|) +76

    |x| +50 = (-26) +76

    |x| +50 = 76 - 26

    |x| +50 = 50

    |x|        = 50 -50

    |x|        = 0

c, |x| + 17 = (-21)+19

    |x| + 17 = - (21-19)

    |x| + 17 = - 2

    |x| không tồn tại

d. |x-21|= 2.(|-11| + |-3|)

    |x-21|= 2.(11+3)

    |x-21|= 2.14

    |x-21|= 28

     x     = 28+21 hoặc 21-28

     x     = 49      hoặc -7

e. hình như lộn đề vì |x-54|-7|

\(\frac{7}{-9}=\frac{-21}{27}=\frac{-42}{54}\)

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