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a/ x2 + 4x - 21= x2 - 3x +4x - 21
= (x2+4x)-(3x+21)
= x(x+4)- 3(x+7)
= (x-3).(x+7)
b/ 3x2-6xy+3y2-3z2 = 3(x2- 2xy+y2- z2)
= 3[(x2 + 2xy + y2) – z2]
= 3[(x + y)2 – z2]
= 3(x + y – z)(x + y + z)
c/ 2x2y + 12xy + 18y = 2y(x2+6x+9)
\(=6\left(4x^2-9y^2+6y-1\right)\)
\(=6\left[\left(2x\right)^2-\left(3y-1\right)^2\right]\)
\(=6\left(2x-3y+1\right)\left(2x+3y-1\right)\)
\(24x^2+36y-54y^2-6\)
\(=6\left(4x^2+6y-9y^2-1\right)\)
\(=6\left[4x^2-\left(9y^2-6y+1\right)\right]\)
\(=6\cdot\left[\left(2x\right)^2-\left(3y-1\right)^2\right]\)
\(=6\left(2x-3y+1\right)\left(2x+3y-1\right)\)
\(a.=x^3+3x^2y+3x^2y+9xy^2+3xy^2+9y^3\)
\(=x^2\left(x+3y\right)+3xy\left(x+3y\right)+3y^2\left(x+3y\right)\)
\(=\left(x+3y\right)\left(x^2+3xy+3y^2\right).\)
\(b.=9x^3+3x^2y+9x^2y+3xy^2+3xy^2+y^3\)
\(=3x^2\left(3x+y\right)+3xy\left(3x+y\right)+y^2\left(3x+y\right)\)
\(=\left(3x^2+3xy+y^2\right)\left(3x+y\right)\).
a) \(2x^2+5x+2\)
\(=2x^2+4x+x+2\)
\(=2x\left(x+2\right)+\left(x+2\right)\)
\(=\left(x+2\right)\left(2x+1\right)\)
b) \(4x^2-4x-9y^2+12y-3\)
\(=\left(4x^2-4x+1\right)-\left(9y^2-12y+4\right)\)
\(=\left(2x-1\right)^2-\left(3y-2\right)^2\)
\(=\left(2x-1+3y-2\right)\left(2x-1-3y+2\right)\)
\(=\left(2x+3y-3\right)\left(2x-3y+1\right)\)
c) \(x^4-2x^3-4x^2+4x-3\)
\(=x^4+x^3-x^2+x-3x^2-3x+3x-3\)
\(=\left(x^4+x^3-x^2+x\right)-\left(3x^2+3x-3x+3\right)\)
\(=x\left(x^3+x^2-x+1\right)-3\left(x^3+x^2-x+1\right)\)
\(=\left(x^3+x^2-x+1\right)\left(x-3\right)\)
d) \(x^3-x+3x^2y+3xy^2+y^3-y\)
\(=\left(x^3+3x^2y+3xy^2+y^3\right)-\left(x+y\right)\)
\(=\left(x+y\right)^3-\left(x+y\right)\)
\(=\left(x+y\right)\left[\left(x+y\right)^2-1\right]\)
\(=\left(x+y\right)\left(x+y-1\right)\left(x+y+1\right)\)
a) \(24x^2-4xy\)
\(=4x\left(6x-y\right)\)
b) \(5x^3-10x^2+5x-20xy^2\)
\(=5x\left(x^2-10x+5-20y^2\right)\)
Ta có
x 3 y 3 + 6 x 2 y 2 + 12 x y + 8 = ( x y ) 3 + 3 ( x y ) 2 . 2 + 3 x y . 2 2 + 2 3 = ( x y + 2 ) 3
Đáp án cần chọn là: A