a) x = 44.             

b) x = 40.            

c) x = 95.             

d) x = 10.

a) x - 120: 30 = 40

    x    -40        =40

              x        =40+40

              x        =80

b) (x + 120) : 20 = 8

    (x+ 120)         = 8x20

     x+120           =160

     x                   = 160-120

     x                  = 40

c) (x + 5). 3 = 300

     x+5=300:3

     x+5=100

     x=100-5

     x=95

d) x.2 + 21 : 3= 27

    x.2   +7=27

      x.2   = 27-7

    x.2= 20

      x=20:2

      x=10

a. ( x + 3 )( x - 3 ) = 16

⇔x²-9=16

⇔x²-16-9=0

⇔x²-25=0

⇔(x-5)(x+5)=0

⇔\(\left[{}\begin{matrix}x+5=0\\x-5=0\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=-5\\x=5\end{matrix}\right.\)

A, 120 x 2 = 120 x 4

120 x 2 = 240

B, số đó là :

(2270 - 1034) : 4 = 309

\(\sqrt{\left(120-11\right)^2}+\sqrt{\left(10-\sqrt{120}\right)^2}\)

\(=120-11+10+\sqrt{120}\)

\(=\sqrt{120}\left(\sqrt{120}+1\right)-1\)

\(a,=\left(120-11\right)+\left|10-\sqrt{120}\right|=109+\sqrt{120}-10=99+2\sqrt{30}\\ b,=\sqrt{\left(\sqrt{x+1}+1\right)^2-\left(\sqrt{x+1}+1\right)^2}=\sqrt{0}=0\)

\(A\left(x\right)=6x^3-x\left(x+2\right)+4\left(x+3\right)\)

\(A\left(x\right)=6x^3-x^2+2x+4x+12\)

\(A\left(x\right)=6x^3-x^2+\left(2x+4x\right)+12\)

\(A\left(x\right)=6x^3-x^2+6x+12\)

\(B\left(x\right)=-x\left(x+1\right)-\left(4-3x\right)+x^2\left(x-2\right)\)

\(B\left(x\right)=-\left(x^2\right)+2-4+3x+x^3-2x^2\)

\(B\left(x\right)=\left(-x^2-2x^2\right)+\left(2-4\right)+3x+x^3\)

\(B\left(x\right)=-3x^2-2+3x+x^3\)

Sửa lại cho Bạn Vũ Đình Phước nhé :v

A (x) = 6x³ – x (x + 2) + 4 (x + 3)

        = 6x³ – x² - 2x + 4x + 12

        = 6x³ – x² + 2x + 12

a) 

1 4 − x = 1 20 + ( − 2 ) 1 4 − x = − 39 20 x = 1 4 − − 39 20 = 5 20 + 39 20 = 44 20 = 11 5

b) 

− 2 5 − x = 4 15 + − 1 3 − 2 5 − x = − 1 15 x = − 2 5 − − 1 15 = − 6 15 − − 1 15 = − 5 15 = − 1 3

a, x= 155 -51 <=> x=104

b.x=-120 : -24 <=> x=5

c.2(81-x)= 155 -35 

<=> 2(81-x)=120

<=> 81 - x = 60

<=> x= 21

Lời giải:

a.

$155-x=51$

$x=155-51=104$

b. 

$(-24)x=-120$

$x=(-120):(-24)=5$

c.

$35+2(81-x)=155$

$2(81-x)=155-35=120$

$81-x=120:2=60$

$x=81-60=21$

a) 155 - x=51

x= 155 - 51

x= 104

b) (-24) . x= -120

x= -120 : -24

x= 5

c) 35 + 2(81 - x) = 155

2(81 - x)= 155 -35

2(81 - x)= 120

81 - x= 120:2

81-x= 60

x= 81 - 60

x= 21

\(\left(2009-x\right):29+32=100\\ =>\left(2009-x\right):29=100-32=68\\ =>2009-x=68.29=1972\\ =>x=2009-1972=37\)

-

\(\left(x-2\right).3-270:45=120\\ =>3\left(x-2\right)-6=120\\ =>3\left(x-2\right)=120+6=126\\ =>x-2=126:3=42\\ =>x=42+2=44\)

\(\frac{x}{3}=\frac{y}{4}\rightarrow x=3k;y=4k\)

Thay x=3k; y=4k vào bt có:

\(2\left(3k\right)^2-3\left(4k\right)^2=-120\)

\(\rightarrow18k^2-48k^2=-120\)

\(\rightarrow-30k^2=-120\)

\(\rightarrow k^2=4\)

\(\Rightarrow k=\left\{2;-2\right\}\)

\(\Rightarrow\left[{}\begin{matrix}x=2.3=6\\y=2.4=8\end{matrix}\right.\) hoặc \(\left[{}\begin{matrix}x=-2.3=-6\\y=-2.4=-8\end{matrix}\right.\)