b) x² + 9x + 20 = x² + 4x + 5x +20 =x(x + 4) + 5(x + 4) = (x + 4)(x+ 5)

a) 7x^2-7y^2-14x+14y= 7 ( x^2 - y^2 ) - 14 ( x+y) = 14 ( 2(x^2) - 2 (x^2))- 28 ( 2x+2y)

= 28 ( 4 ( x^2)-4(y^2))- 28 ( 2x+2y)

= 28 (4(x^2-y^2) - 4(x+y)

= 28 (4( x^2-y^2-x+y ))

= 112 . ( x-y-x+y)

lớp 8 hả thôi mình bó tay

M=6x-x²+2

= -(x² - 6x + 3²)+ 3² +2

= -( x - 3)² + 11

Ta có: -( x - 3)² ≥ 0 vs mọi x

<=> -( x - 3)² + 11 ≥11

Dấu ''='' xảy ra <=> -(x-3) = 0

=> -x + 3 = 0

=> x = 3

Vậy vs x = 3 thì min M = 11

A) 7X² - 7Y² - 14X + 14Y

= ( 7X² - 7Y² ) - ( 14X - 14Y )

= 7( X² - Y² ) - 14( X - Y )

= 7( X - Y )( X + Y ) - 14( X - Y )

= 7( X - Y )( X + Y - 14 )

B) X² - Y² + 14X + 49

= ( X² + 14X + 49 ) - Y²

= ( X + 7 )² - Y²

= ( X - Y + 7 )( X + Y + 7 )

C) X² - Y² - X + Y

= ( X² - Y² ) - ( X - Y )

= ( X - Y )( X + Y ) - ( X - Y )

= ( X - Y )( X + Y - 1 )

D) X² + 12Y - Y² - 36

= X² - ( Y² - 12Y + 36 )

= X² - ( Y - 6 )²

= ( X - Y + 6 )( X + Y - 6 )

E) X³ + X² - 9X - 9

= ( X³ + X² ) - ( 9X + 9 )

= X²( X + 1 ) - 9( X + 1 )

= ( X + 1 )( X² - 9 )

= ( X + 1 )( X - 3 )( X + 3 )

sửa ý a) thành 7( x - y )( x + y - 2 ) nhé ;-;

1)x²+10xy+25y²=(x+5y)²

2)7x²-7y²-14x+14y=7(x-y)(x+y)-14(x-y)

=(x-y)(7x+7y-14)

=7(x-y)(x-y-2)

3)x²-2xy-4+y²=x²-2xy+y²-4=(x-y)²-4=(x-y-4)(x-y+4)

4)x²+11x+24=x²+2.\(\frac{11}{2}x\)+\(\left(\frac{11}{2}\right)^2\)-\(\frac{25}{4}\)

=(x+\(\frac{11}{2}\))²-\(\left(\frac{5}{2}\right)^2\)

=\(\left(x+3\right)\left(x+\frac{17}{2}\right)\)

Trả lời:

7x² - 7y² - 14x + 14y 

= 7 ( x² - y² - 2x + 2y )

= 7 [ ( x² - y² ) - ( 2x - 2y ) ] 

= 7 [ ( x - y )( x + y ) - 2 ( x - y ) ]

= 7 ( x - y )( x + y - 2 )

a) thay x,y=3 ta đc

A=7\(\times\)3 + 7\(\times\)3

  =7\(\times\)(3+3)

  =7\(\times\)6

  = 42

b)thay x,y=3 ta có :

B=10\(\times\)3+14\(\times\)3+4\(\times\)3

  =3\(\times\)(10+14+4)

 =3\(\times\)28

 =84

\(7x^3+14x^2+7x+7y^2\)

\(=7\left(x^3+2x^2+x+y^2\right)\)

\(g,x^2-2xy+y^2-9z^2=\left(x-y\right)^2-\left(3z\right)^2\)\(=\left(x-y+3z\right)\left(x-y-3z\right)\)

\(h,5x^4-20x^2=5x^2\left(x^2-4\right)=5x^2\left(x-2\right)\left(x+2\right)\)

\(i,7x^2-7y^2-14x+14y=7\left(x-y\right)\left(x+y\right)-14\left(x-y\right)\)

\(=\left(x-y\right)\left(7x+7y-14\right)=7\left(x-y\right)\left(x+y-2\right)\)

\(k,x^2+8x+3x+24=x\left(x+8\right)+3\left(x+8\right)=\left(x+8\right)\left(x+3\right)\)

\(m,x^4-y^4=\left(x^2-y^2\right)\left(x^2+y^2\right)=\left(x-y\right)\left(x+y\right)\left(x^2+y^2\right)\)

\(n,x^6-y^6=\left(x^2-y^2\right)\left(x^4+x^2y^2+y^4\right)=\left(x-y\right)\left(x+y\right)\left(x^4+x^2y^2+y^4\right)\)