\(B=4x-4y+5xy=4.\left(x-y\right)+5xy\)

Thay \(x-y=\frac{5}{12};xy=-\frac{1}{3}\)vào B ta có 

\(B=4.\frac{5}{12}+5.\frac{-1}{3}=\frac{20}{12}+\frac{-5}{3}=0\)

Vậy B = 0

B=4x-4y+5xy

B=4.5/12+5.-1/3

B=4.5/12+1+4.-1/3

B=4.(5/12+-1/3)+1

B=4.1/12+1

B=1/3+1

B=4/3

Thay x =-1 và y= 1 vào biểu thức B ta có

B = \(6.\left(-1\right).1+4.\left(-1\right)^4\)\(-1^7\)-\(4.\left(-1\right)^4\).\(1^3\) \(+10-5.\left(-1\right).1+2.1^7\)-\(5\)

B = \(6.\left(-1\right).1+4.1-1-4.1+10-5.\left(-1\right).1+2.1-5\)

B = 5

b: \(B=\dfrac{3y+5}{y-1}-\dfrac{-y^2-4y}{y-1}+\dfrac{y^2+y+7}{y-1}\)

\(=\dfrac{3y+5+y^2+4y+y^2+y+7}{y-1}\)

\(=\dfrac{2y^2+8y+12}{y-1}\)

a, \(5ax-15ay+20a=5a\left(x-5y+4\right)\)

b, sai 

c, \(3ab\left(x+y\right)+3a\left(y-x\right)=3ab\left(x+y\right)-3a\left(x+y\right)=\left(3ab-3a\right)\left(x+y\right)\)

d, \(x^2-xy+2x-2y=x\left(x+2\right)-y\left(x+2\right)=\left(x-y\right)\left(x+2\right)\)

Tượng tự ... 

a) 5ax - 15ay + 20a = 5a( x - 3y + 4 )

b) 6xy - 12x - 8y = 2( xy - 6x - 4y )

c) 3ab( x - y ) + 3a( y - x ) = 3ab( x - y ) - 3a( x - y ) = ( x - y )( 3ab - 3a ) = 3a( x - y )( b - 1 )

d) x² - xy + 2x - 2y = x( x - y ) + 2( x - y ) = ( x - y )( x + 2 )

e) ax² - 5x² - ax + 5x + a - 5 = x²( a - 5 ) - x( a - 5 ) + ( a - 5 ) = ( a - 5 )( x² - x + 1 )

g) x²y - 4xy² + 4y³ - 36yz² = y( x² - 4xy + 4y² - 36z² ) = y[ ( x² - 4xy + 4y² ) - 36z² ] = y[ ( x - 2y )² - ( 6z )² ] = y( x - 2y - 6z )( x - 2y + 6z )

h) 4xy - x² - 4y² + m² - 6m + 9

= ( m² - 6x + 9 ) - ( x² - 4xy + 4y² )

= ( m - 3 )² - ( x - 2y )²

= ( m - 3 - x + 2y )( m - 3 + x - 2y )

i) x² + x - 12 = x³ - 3x + 4x - 12 = x( x - 3 ) + 4( x - 3 ) = ( x - 3 )( x + 4 )

k) 5x² + 14x - 3 = 5x² - x + 15x - 3 = x( 5x - 1 ) + 3( 5x - 1 ) = ( 5x - 1 )( x + 3 )

m) x² - 5xy + 4y² = x² - xy - 4xy + 4y² = x( x - y ) - 4y( x - y ) = ( x - y )( x - 4y ) < đã sửa đề >

n) 3x² - 5xy + 2y² + 4x - 4y = ( 3x² - 5xy + 2y² ) + ( 4x - 4y ) = ( 3x² - 3xy - 2xy + 2y² ) + 4( x - y ) = [ 3x( x - y ) - 2y( x - y ) ] + 4( x - y ) = ( x - y )( 3x - 2y ) + 4( x - y ) = ( x - y )( 3x - 2y + 4 )

f) 2x³ + 4x²y + 2xy² = 2x( x² + 2xy + y² ) = 2x( x + y )²

a) \(5ax-15ay+20a\)

\(=5a\left(x-3y+4\right)\)

b) \(6xy-12x-8y\)

\(=6\left(xy-2x-3y\right)\)

c) \(3ab\left(x-y\right)+3a\left(y-x\right)\)

\(=3a\left(x-y\right)\left(b-1\right)\)

d) \(x^2-xy+2x-2y\)

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

e) \(ax^2-5x^2-ax+5x+a-5\)

\(=\left(a-5\right)\left(x^2-x+1\right)\)

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

2, \(x^2+3x-10=x^2+5x-2x-10=x\left(x-2\right)+5\left(x-2\right)=\left(x+5\right)\left(x-2\right)\)

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

4, \(3x^2+4x-7=3x^2+7x-3x-7=3x\left(x-1\right)+7\left(x-1\right)=\left(3x+7\right)\left(x-1\right)\)

5, \(4x^2-9y^2-5xy=4x^2-9xy+4xy-9y^2\)

\(=4x\left(x+y\right)-9y\left(x+y\right)=\left(4x-9y\right)\left(x+y\right)\)

6, \(x^2-2x-4y^2-4y=x^2-2x+1-4y^2-4y-1=\left(x-1\right)^2-\left(2y+1\right)^2\)

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