Ta có : 

\(3-2\left(8x-3\right)-9x^2-12x+4x^2\)

\(=3-16x+6-9x^2-12x+4x^2=-5x^2+9-28x\)

Thay x = -2 vào biểu thức trên ta được : 

\(-5\left(-2\right)^2+9-28\left(-2\right)=-20+9+56=-11+56=45\)

a)  \(8x^3+125=\left(2x+5\right)\left(4x^2-10x+25\right)\)

b) \(x^2+2y-y^2-1=x^2-\left(y-1\right)^2=\left(x-y+1\right)\left(x+y-1\right)\)

c) \(-4x^2-25y^2+20xy+36=36-\left(2x-5y\right)^2=\left(6-2x+5y\right)\left(6+2x-5y\right)\)

Bài 4.

a) 3xy² - 45x²y = 3xy( y - 15x )

b) 25y² - 4x² + 4x - 1

= 25y² - ( 4x² - 4x + 1 )

= ( 5y )² - ( 2x - 1 )²

= ( 5y - 2x + 1 )( 5y + 2x - 1 )

c) x² - 5x + xy - 5y

= x( x - 5 ) + y( x - 5 )

= ( x - 5 )( x + y )

d) x² - 8x - 33

= x² + 3x - 11x - 33

= x( x + 3 ) - 11( x + 3 )

= ( x + 3 )( x - 11 )

Bài 5.

a) A = ( x - 2 )³ - x²( x - 4 ) + 8

= x³ - 6x² + 12x - 8 - x³ + 4x² + 8

= -2x² + 12x

B = ( x² - 6x + 9 ) : ( x - 3 ) - x( x + 7 ) - 9

= ( x - 3 )² : ( x - 3 ) - x² - 7x - 9

= x - 3 - x² - 7x - 9

= -x² - 6x - 12

b) Với x = -1 thì A = -2.(-1)² + 12.(-1) = -2 - 12 = -14

a. x² + 6x + 9 = (x + 3)²

b. 25 + 10x + x² = (5 + x)²

c. x² + 8x + 16 = (x + 4)²

d. x² + 14x + 49 = (x + 7)²

e. 4x² + 12x + 9 = (2x + 3)²

f. 9x² + 12x + 4 = (3x + 2)²

h. 16x² + 8 + 1 = (4x + 1)²

i. 4x² + 12xy + 9y² = (2x + 3y)²

k. 25x² + 20xy + 4y² = (5x + 2y)²

a) \(=\left(x+3\right)^2\)

b) \(=\left(x+5\right)^2\)

c) \(=\left(x+4\right)^2\)

d) \(=\left(x+7\right)^2\)

e) \(=\left(2x+3\right)^2\)

f) \(=\left(3x+2\right)^2\)

h) \(=\left(4x+1\right)^2\)

i) \(=\left(2x+3y\right)^2\)

k) \(=\left(5x+2y\right)^2\)

`a, A= 4xy -xy-2xy`

`= (4-1-2)xy`

`= xy`

Thay `x=2;y=3`

Ta có : `xy=2*3=6`

`b, B= x^2 y -7x^2y-4x^2y`

`=(1-7-4)x^2y`

`= -10x^2y`

Thay `x=2;y=3`

Ta có : `-10x^2y=-10*2^2 *3= -10*4*3=-40*3=-120`

`c, C=10x^2y -x^2y-7x^2y`

`=(10-1-7)x^2y`

`= 2x^2y`

Thay `x=2;y=3`

Ta có : `2x^2y=2*2^2 *3= 2*4*3=8*3=24`

`d,D=5x^2y^2-12x^2y^2+8x^2y^2`

`= (5-12+8)x^2y^2`

`=x^2y^2`

Thay `x=2;y=3`

ta có : `x^2y^2=2^2 *3^2= 4* 9=36`

có chỗ nào bn đọc ko rõ thì ns mik nha, để mik gõ ra cho bn rõ hơn

\(4x^2-20xy^2+25y^4=\left(2x\right)^2-2.2x.5y^2+\left(5y^2\right)^2=\left(2x-5y^2\right)^2\)

Áp dụng hằng đẳng thức: \(\left(A-B\right)^2=A^2-2AB+B^2\)

\(4x^2-20xy^2+25y^4\)

\(=\left(2x\right)^2-2\cdot2x\cdot5y^2+\left(5y\right)^2\)

\(=\left(2x-5y\right)^2\)

Ta có :  6 - 3x³ + 2x²  - x³ + 7x + 12x³  -8x 

( -3x³  - x³ + 12x³ ) +  2x² + ( 7x - 8x ) + 6

8x³  + 2x²  - 1x + 6

chúc bn học tốt !~@#

\(A=\)\(\frac{x|x-2|}{x^2+8x-20}+12x-3.\)

\(=\frac{x|x-2|}{\left(x-2\right)\left(x+10\right)}+12x-3\)

Nếu \(x\ge2\Rightarrow x-2\ge0\Leftrightarrow|x-2|=x-2\)

\(\Rightarrow A=\frac{x\left(x-2\right)}{\left(x-2\right)\left(x+10\right)}+12x-3=\frac{x}{x+10}+12x-3\)

Nếu \(x< 2\Rightarrow x-2< 0\Leftrightarrow|x-2|=-\left(x-2\right)\)

\(\Rightarrow A=\frac{-x\left(x-2\right)}{\left(x-2\right)\left(x+10\right)}+12x-3=\frac{-x}{x+10}+12x-3\)

Cảm ơn bạn