1) \(\Rightarrow\dfrac{x}{8}=\dfrac{y}{12}=\dfrac{z}{15}\)

Áp dụng t/c dtsbn:

\(\dfrac{x}{8}=\dfrac{y}{12}=\dfrac{z}{15}=\dfrac{x-y+z}{8-12+15}=\dfrac{10}{11}\)

\(\Rightarrow\left\{{}\begin{matrix}\dfrac{x}{8}=\dfrac{10}{11}\\\dfrac{y}{12}=\dfrac{10}{11}\\\dfrac{z}{15}=\dfrac{10}{11}\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=\dfrac{80}{11}\\y=\dfrac{120}{11}\\z=\dfrac{150}{11}\end{matrix}\right.\)

2) \(\Rightarrow\left\{{}\begin{matrix}\dfrac{x}{3}=\dfrac{y}{4}\\\dfrac{y}{5}=\dfrac{z}{7}\end{matrix}\right.\) \(\Rightarrow\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}\)

Áp dụng t/c dtsbn:

\(\dfrac{x}{15}=\dfrac{y}{20}=\dfrac{z}{28}=\dfrac{2x}{30}=\dfrac{3y}{60}=\dfrac{2x+3y-z}{30+60-28}=\dfrac{136}{62}=\dfrac{68}{31}\)

\(\Rightarrow\left\{{}\begin{matrix}\dfrac{x}{15}=\dfrac{68}{31}\\\dfrac{y}{20}=\dfrac{68}{31}\\\dfrac{z}{28}=\dfrac{68}{31}\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=\dfrac{1020}{31}\\y=\dfrac{1360}{31}\\z=\dfrac{1904}{31}\end{matrix}\right.\)

3) \(\Rightarrow\dfrac{3x-9}{15}=\dfrac{5y-25}{5}=\dfrac{7z+21}{49}\)

Áp dụng t/c dtsbn:

\(\dfrac{3x-9}{15}=\dfrac{5y-25}{5}=\dfrac{7z+21}{49}=\dfrac{3x+5y-7z-9-25-21}{15+5-49}=-\dfrac{45}{29}\)

\(\Rightarrow\left\{{}\begin{matrix}\dfrac{3x-9}{15}=-\dfrac{45}{29}\\\dfrac{5y-25}{5}=-\dfrac{45}{29}\\\dfrac{7z+21}{49}=-\dfrac{45}{29}\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x=-\dfrac{138}{29}\\y=\dfrac{100}{29}\\z=-\dfrac{402}{29}\end{matrix}\right.\)

\(\frac{x}{2}=\frac{y}{3}\\ \Rightarrow\frac{x}{10}=\frac{y}{15}\)

\(\frac{y}{5}=\frac{z}{7}\\ \Rightarrow\frac{y}{10}=\frac{z}{14}\)

\(\Rightarrow\frac{x}{10}=\frac{y}{15}=\frac{z}{14}=\frac{2x}{20}=\frac{3y}{45}=\frac{z}{14}=\frac{2x+3y+z}{20+45+14}=\frac{102}{79}\)

\(\Rightarrow x=\frac{1020}{79};y=\frac{1530}{79};z=\frac{1428}{79}\)

suy ra:  x/10 = y/15  ;   y/15 = z/21 và 2x +3y +z =102

suy ra: x/10 = y/15 = z/21 và  2x +3y +z =102

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

x/10=y/15=z/21 = 2x/20 = 3y/45 = z/21 = 2x+3y+z / 20 +45 +21 = 102/86 = 51/43 

SAI ĐỀ RÙI

1)\(2x-5=-17\)

\(\Leftrightarrow2x=-12\)

\(\Leftrightarrow x=-6\)

2) \(4\left(x+41\right)=400\)

\(\Leftrightarrow x+41=100\)

\(\Leftrightarrow x=59\)

720 : [ 41 - ( 2x - 5 ) ] = 2³ . 5

720 : [ 41 - ( 2x - 5 ) ] = 40

 [ 41 - ( 2x - 5 ) ]  = 720 : 40

  41 - ( 2x - 5 ) = 18

2x - 5 = 41 - 18

2x - 5 = 23

2x = 23  + 5

2x = 28

x = 28 : 2

x = 14

Vậy x = 14

a. 720 : [ 41 - ( 2x - 5 ) ] = 2³ . 5

=> 720 : [ 41 - (2x-5) ] = 8.5

=> 720 : [ 41 - (2x-5) ] = 40

=> 41 - (2x-5) = 720 : 40

=> 41 - (2x-5) = 18

=> 2x - 5 = 41 - 18

=> 2x - 5 = 23

=> 2x = 23 + 5

=> 2x = 28

=> x = 28 : 2

=> x = 14 thuộc Z

vậy______

b. ( 3x - 2⁴ ) . 7⁴ = 2.7⁵

=> 3x-16 = 2.7⁵ : 7⁴

=> 3x - 16 = 2.7

=> 3x - 16 = 14

=> 3x = 14 + 16

=> 3x = 30

=> x = 30 : 3

=> x = 10 thuộc Z

vậy_____

( 3x - 2⁴ ) . 7⁴ = 2 . 7⁵

3x  - 16 = 2 . 7⁵ : 7⁴

3x - 16 = 2 . 7

3x - 16 = 14

3x = 14 + 16

3x = 30

x = 30 : 3

x = 10

Vậy x = 10

2x-5=-17

2x   = -17+5

2x   =  -12

  x   = -12 : 2

  x   = -6

4.(x+41)=400

   (x+41)=400:4

    x+41 = 100

    x       = 100 - 41

    x       59

a) 10-x-5=-5-7-11

=> 5 - x = -23

=> x = 28
b) |x| -3=0

=> |x| = 3

=> x = 3 hoặc x -3
c) ( 7-|x| ) .(2x-4)=0

=> 7 - |x| = 0 hoặc 2x - 4 = 0

=> |x| = 7 hoặc 2x = 4

=> x = 7 hoặc x = - 7 hoặc x = 2
c)2+3x=-15-19 

=> 2 + 3x = -34

=> 3x = 36

=> x = 12 
 

\(a,10-x-5=-5-7-11\)

\(10-x-5=-23\)

\(10-x=-18\)

\(x=28\)

\(b,|x|-3=0\)

\(|x|=3\)

\(\Rightarrow\orbr{\begin{cases}x=3\\x=-3\end{cases}}\)

Tìm x thuoc z:

1) \(26-\left|x+9\right|=-13\)

\(\Leftrightarrow\left|x+9\right|=26-\left(-13\right)\)

\(\Leftrightarrow\left|x+9\right|=39\)

\(\Leftrightarrow\left[{}\begin{matrix}x+9=39\\x+9=-39\end{matrix}\right.\)

\(\Leftrightarrow\left[{}\begin{matrix}x=39-9=30\\x=-39-9=-48\end{matrix}\right.\)

Vậy: \(x\in\left\{30;-48\right\}\)

2) \(\left|x+7\right|-13=25\)

\(\Leftrightarrow\left|x+7\right|=25+13=38\)

\(\Leftrightarrow x+7\in\left\{38;-38\right\}\)

\(\Leftrightarrow x\in\left\{31;-45\right\}\)

Vậy:.................

tim x biet

\(1)123-3.\left(x+4\right)=23\)

\(\Leftrightarrow3\left(x+4\right)=123-23\)

\(\Leftrightarrow3\left(x+4\right)=100\)

\(\Leftrightarrow x+4=\frac{100}{3}\)

\(\Leftrightarrow x=\frac{100}{3}-4=\frac{100-12}{3}=\frac{88}{3}\)

Vậy:................

2) Tương tự