( 30 - 3x ) . 7^2 = 3 . 7^3

( 30 - 3x ) . 49  = 3 . 343

( 30 - 3x ) . 49  = 1029

30 - 3x             = 1029 : 49 

30 - 3x            = 21

        3x           = 30 - 21

         3x          = 9

          x           = 9 : 3

          x           = 3 

(30-3x).7²=3.7²

(30-3x).7²=147

30-3x=147:7²

30-3x=3

3x=30-3

3x=27

x=27:3

x=9

( 30 - 3x ) . 7^2 = 3 . 7^3

( 30 - 3x ) . 49 = 3 . 343

( 30 - 3x ) . 49 = 1029

30 - 3x           = 1029 : 49

30 - 3x          = 21

       3x         = 30 - 21

        3x         = 9

         x         = 9 : 3

          x        = 3

(30-3x).7²=3.7²

(30-3x).7²=147

30-3x=147:7²

30-3x=3

3x=30-3

3x=27

x=27:3

x=9

x=3

tick nhecông chúa họ thạch

\(2^5.x+3y=32x+3y⋮7\)

Ta có

\(35x+14y⋮7\)

\(\Rightarrow\left(35x+14y\right)-\left(32x+3y\right)=3x+11y⋮7\)

(x-1²)=16
=> x-1=16
=> x=16+1
=> x=17

`(x-1^2)=16`

`x-1=16`

`x=16+1=17`

__________________________________

Hoặc có thể đề là vầy:

 `(x-1)^2=16`

`@TH1:(x-1)^2=4^2`

      `=>x-1=4`

      `=>x=5`

`@TH2:(x-1)^2=(-4)^2`

        `=>x-1=-4`

        `=>x=-3`

\(2x+7=2x+2+5=2\left(x+1\right)+5⋮x+1\\ =>x+1\inƯ\left(5\right)\\ Ư\left(5\right)=\left\{1;-1;5;-5\right\}\\ \left\{{}\begin{matrix}x+1=1\\x+1=-1\\x+1=5\\x+1=-5\end{matrix}\right.=>\left\{{}\begin{matrix}x=0\\x=-2\\x=4\\x=-6\end{matrix}\right.\)

Bài 6:

\(21,251+6,058+0,749+1,042\)

\(=\left(21,251+0,749\right)+\left(6,058+1,042\right)\)

\(=22+7,1\)

\(=29,1\)

___________________

\(1,53+5,309+12,47+5,691\)

\(=\left(1,53+12,47\right)+\left(5,309+5,691\right)\)

\(=14+11\)

\(=25\)

5:

a: =>x/17=5/17

=>x=5

b; =>6+x=7/11*33=21

=>x=15

c: \(\dfrac{12+x}{43-x}=\dfrac{2}{3}\)

=>3x+36=86-2x

=>5x=50

=>x=10

d: \(\dfrac{x}{5}< \dfrac{3}{7}\)

=>x<3/7*5

=>x<15/7

f: 15/26+x/16=46/52

=>x/16=23/26-15/26=8/26=4/13

=>x=4/13*16=64/13

-12.(x - 5) + 7.(3 - x) = 5

=> -12x + 60 + 21 - 7x = 5

=> -12x - 7x = 5 - 21 - 60

=> -19x = -76

=> x = 4

Nhân phân phối vào là giải được

-12(x-5) + 7(3-x) = 5

-12x + 60 + 21 - 7x = 5

-12x - 7x = 5 - 60 -21

(-12 -7)x = - 76

-19x = -76

x= - 76 :(-19)

x= 4

a, 100 - 7 x ( x - 5 ) = 58

7 x ( x - 5 ) = 100 - 58

7 x ( x - 5 ) = 42

( x - 5 ) = 42 : 7

x - 5 = 6

x = 6 + 5

x = 11

b, 24 + 5 * x = 49

5 * x = 49 - 24

5 * x = 25

x = 25 : 5

x = 5

c, x - 105 : 21 = 15

x - 5 = 15

x = 15 + 5

x = 20

d, ( x - 105 ) : 21 = 15

( x - 105 ) = 15 x 21

x - 105 = 315

x = 315 + 105

x = 420

3. tìm số tự nhiên x biết 

a 100 - 7x [ x - 5] = 58

b 24+5 nhân x = 49

c x- 105 : 21 = 15

d { x- 105] : 21 =15

a, 100 - 7 x ( x - 5 ) = 58

7 x ( x - 5 ) = 100 - 58

7 x ( x - 5 ) = 42

( x - 5 ) = 42 : 7

x - 5 = 6

x = 6 + 5

x = 11

b, 24 + 5 * x = 49

5 * x = 49 - 24

5 * x = 25

x = 25 : 5

x = 5

Áp dụng bất đẳng thức Cosi ta có:

\(x+y\ge2\sqrt{xy}\)

\(\Rightarrow2\sqrt{S}\le12\Leftrightarrow\sqrt{S}\le6\Rightarrow S\le36\)

Dấu = xảy ra khi x=y=6