a: x=2/5*15=6

b: =>x-7=15

=>x=22

c: =>3/4:x+1/2*4=4

=>3/4:x=4-2=2

=>x=3/8

a) [ 25 - ( x + 1,85 ) ] : 3 = 7

[ 25 - ( x + 1,85) ] = 7x3

[ 25 - ( x + 1,85 ) ] = 21

x + 1,85= 25-21

x + 1,85= 4

x= 4-1,85

x= 2,15

a) [25 - (x + 1,85)] : 3 = 7

=> 25 - (x + 1,85)       = 3 x 7

=> 25 - (x + 1,85)       = 21

=> x + 1,85                 = 25 - 21

=> x + 1,85                  = 4

=> x                             = 4 - 1,85

=> x                             = 2,15

b) 420 : [75  - (x - 10)] = 21

=> 75 - (x - 10) = 420 : 21

=> 75 - (x - 10) = 20

=> x - 10           = 75 - 20

=> x - 10           = 55

 => x                  = 55 + 10

=> x                   = 65

b) 420 : [ 75 - ( x-10 ) ] = 21

[ 75 - ( x-10) ]= 420:21

[ 75 - ( x-10) ] = 20

( x- 10) =75-20

 x-10 = 55

x= 55+10

x=65

420:[75-(x-10)]=21

=> 75 - ( x-10) = 420 : 21 = 20 

=> x -10 = 75 -20 = 55 

=> x = 55 + 10 = 65

420 : [75 - (x - 10)] = 21

75 - (x - 10) = 420 : 21

75 - (x - 10) = 20

x - 10 = 75 - 20

x - 10 = 55

x = 55 + 10

x = 65

Vậy x = 65

\(420:\left[75-\left(x-10\right)\right]=21\)

\(75-\left(x-10\right)=420:21\)

\(75-\left(x-10\right)=20\)

\(x-10=75-20\)

\(x-10=55\)

\(x=55+10\)

\(x=65\)

420 : [ 75 - ( x - 10) ] = 21

           75 - ( x - 10 ) = 420 : 21 

           75 - ( x - 10 ) =  20 

                      x - 10 = 75 - 20

                        x -10 = 55 

                        x       = 55 + 10 

                          x      = 65 

x = 65 nha đúng 100%

Ta có: 420: (75-(x-10))= 21 => 75-(x-10)=20 => x-10=55 => x=65. Vậy, x=65

a: \(\left(x-4\right)^2-\left(x-3\right)\left(x+3\right)=5\)

\(\Leftrightarrow x^2-8x+16-x^2+9=5\)

\(\Leftrightarrow-8x=-20\)

hay \(x=\dfrac{5}{2}\)

A)    \(x-\dfrac{2}{3}=\dfrac{4}{5}\\ x=\dfrac{4}{5}+\dfrac{2}{3}\)

           \(x=\dfrac{22}{15}\)

b)\(\dfrac{7}{9}-x=\dfrac{1}{3}\\ x=\dfrac{7}{9}-\dfrac{1}{3}\\ x=\dfrac{4}{9}\)

C)\(x:\dfrac{2}{3}=\dfrac{9}{8}\\ x=\dfrac{9}{8}x\dfrac{2}{3}\\ x=\dfrac{3}{4}\)

a, \(\dfrac{4x}{7}=\dfrac{1}{5}+\dfrac{2}{3}=\dfrac{3+10}{15}=\dfrac{13}{15}\Rightarrow60x=91\Leftrightarrow x=\dfrac{91}{60}\)

b, \(\dfrac{5}{7}:x=\dfrac{1}{6}+\dfrac{4}{5}=\dfrac{5+24}{30}=\dfrac{29}{30}\Leftrightarrow x=\dfrac{5}{7}:\dfrac{29}{30}=\dfrac{150}{203}\)

Bài 3: 

a: Ta có: 60-3(x-2)=51

\(\Leftrightarrow x-2=3\)

hay x=5

b: Ta có: \(4x-20=25:2^2\)

\(\Leftrightarrow4x=\dfrac{25}{4}+20=\dfrac{105}{4}\)

hay \(x=\dfrac{105}{16}\)

c: Ta có: \(8\cdot6+288:\left(x-3\right)^2=50\)

\(\Leftrightarrow288:\left(x-3\right)^2=50-48=2\)

\(\Leftrightarrow\left(x-3\right)^2=144\)

\(\Leftrightarrow\left[{}\begin{matrix}x-3=12\\x-3=-12\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=15\\x=-9\end{matrix}\right.\)

a: \(\Leftrightarrow\left[{}\begin{matrix}x-\dfrac{3}{4}=\dfrac{1}{5}\\x-\dfrac{3}{4}=-\dfrac{1}{5}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{19}{20}\\x=\dfrac{11}{20}\end{matrix}\right.\)

\(a,\left|x-\dfrac{3}{4}\right|=\dfrac{1}{5}\)

\(\Leftrightarrow\left[{}\begin{matrix}x-\dfrac{3}{4}=\dfrac{1}{5}\\x-\dfrac{3}{4}=\dfrac{-1}{5}\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{19}{20}\\x=\dfrac{11}{20}\end{matrix}\right.\)

\(b,\dfrac{-1}{3}+\left|x\right|=0,5\)

\(\Leftrightarrow\left|x\right|=\dfrac{5}{6}\Leftrightarrow\left[{}\begin{matrix}x=\dfrac{5}{6}\\x=\dfrac{-5}{6}\end{matrix}\right.\)