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a) Ta có:
x³ + y³ + z³ - 3xyz = (x+y)³ - 3xy(x-y) + z³ - 3xyz
= [(x+y)³ + z³] - 3xy(x+y+z)
= (x+y+z)³ - 3z(x+y)(x+y+z) - 3xy(x-y-z)
= (x+y+z)[(x+y+z)² - 3z(x+y) - 3xy]
= (x+y+z)(x² + y² + z² + 2xy + 2xz + 2yz - 3xz - 3yz - 3xy)
= (x+y+z)(x² + y² + z² - xy - xz - yz).
= (x + y)3 + z3 – 3x2y – 3xy2 - 3xyz
= (x + y +z)[(x + y)2 – (x + y)z + z2)] - 3xy(x + y + z)
= (x + y + z)(x2 +2xy + y2 – xz – yz +z2 – 3xy)
= (x + y + z)(x2 + y2 +z2 – xy - yz – xz)
\(x^3+y^3+z^3-3xyz\)
\(=\left(x+y\right)^3+z^3-3x^2y-3xy^2-3xyz\)
\(=\left(x+y\right)^3-3xy\left(x-y\right)+z^3-3xyz\)
\(=\left[\left(x+y\right)^3+z^3\right]-3xy\left(x+y+z\right)\)
\(=\left(x+y+z\right)^3-3z\left(x+y\right)\left(x+y+z\right)-3xy\left(x-y-z\right)\)
\(=\left(x+y+z\right)\left[\left(x+y+z\right)^2-3z\left(x+y\right)-3xy\right]\)
\(=\left(x+y+z\right)\left(x^2+y^2+z^2+2xy+2xz+2yz-3xz-3yz-3xy\right)\)
\(=\left(x+y+z\right)\left(x^2+y^2+z^2-xy-xz-yz\right)\)
1. 8 - 12x + 6x2 - x3
= 23 - 3.22.x + 3.x2.2 - x3
=(2-x)3
2. 125x3 - 75x2 +15x - 1
=(5x)3 - 3.(5x)2.1 + 3.5x.12 - 13
=(5x - 1)3
3, 4 (sai đề)
5. x3 + 2x2 - 6x - 27
=(x3 - 27) + (2x2 - 6x)
=(x3 - 33) + (2x2 - 6x)
=(x -3)(x2 + 3x + 9) + 2x(x-3)
=(x-3)(x2 + 3x +9 +2x)
=(x-3)(x2 + 5x +9)
6. 12x3 + 4x2- 27x -9
=(12x3 + 4x2) - (27x + 9)
=4x2(3x + 1) - 9(3x +1)
=(3x -1)(4x2 -9)
=(3x-1)(2x-3)(2x+3)
Bài 1 :
\(=\left(x^3-x\right)-\left(6x+6\right)\)
\(=x\left(x^2-1\right)-6\left(x+1\right)\)
\(=x\left(x+1\right)\left(x-1\right)-6\left(x+1\right)\)
\(=\left(x^2-x\right)\left(x+1\right)-6\left(x+1\right)\)
\(=\left(x^2-x-6\right)\left(x+1\right)\)
a) \(2\left(x-y\right)\left(x+y\right)+\left(x+y\right)^2+\left(x-y\right)^2\)
\(\left(x+y+x-y\right)^2=\left(2x\right)^2=4x^2\)
b) \(\left(x-y+z\right)^2+\left(z-y\right)^2+2\left(x-y+z\right)\left(y-z\right)\)
\(=\left(x-y+z\right)^2+\left(y-z\right)^2+2\left(x-y+z\right)\left(y-z\right)\)
\(=\left(x-y+z+y-z\right)^2=x^2\)
a) \(\left(x-y+x+y\right)^2=4x^2\)
b) \(\left(x-y+z+y-z\right)^2=x^2\)
a)\(\left(x^2+x\right)^2+4\left(x^2+x\right)-12\)
\(=\left(x^2+x+4\right)\left(x^2+x\right)-12\)
Đặt \(t=x^2+x\) ta có:
\(\left(t+4\right)t-12=t^2+4t-12\)
\(=\left(t-2\right)\left(t+6\right)=\left(x^2+x-2\right)\left(x^2+x+6\right)\)
\(=\left(x-1\right)\left(x+2\right)\left(x^2+x+6\right)\)
b)\(x^8+x+1\)
\(=x^8-x^2+\left(x^2+x+1\right)\)
\(=x^2\left(x^6-1\right)+\left(x^2+x+1\right)\)
\(=x^2\left(x^3+1\right)\left(x^3-1\right)+\left(x^2+x+1\right)\)
\(=x^2\left(x^3+1\right)\left(x-1\right)\left(x^2+x+1\right)+\left(x^2+x+1\right)\)
\(=\left(x^2+x+1\right)\left[x^2\left(x^3+1\right)\left(x-1\right)+1\right]\)
\(x^4+2002x^2+2001x+2002\)
\(=x^4+x^2+1+2001x^2+2001x+2001\)
\(=\left(x^4+2x^2+1\right)-x^2+2001\left(x^2+x+1\right)\)
\(=\left(x^2+1-x\right)\left(x^2+1+x\right)+2001\left(x^2+x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^2+1-x+2001\right)\)
\(=\left(x^2+x+1\right)\left(x^2-x+2002\right)\)
\(x^4+2007x^2-2006x+2007\)
\(=x^4+2x^2+1-x^2+2006\left(x^2-x+1\right)\)
\(=\left(x^2+1\right)^2-x^2+2006\left(x^2-x+1\right)\)
\(=\left(x^2+1+x\right)\left(x^2+1-x\right)+2006\left(x^2-x+1\right)\)
\(=\left(x^2-x+1\right)\left(x^2+x+1+2006\right)\)
\(=\left(x^2-x+1\right)\left(x^2+x+2007\right)\)
2(x4+y4+z4)-(x2+y2+z2)2-2(x2+y2+z2)(x+y+z)2+(x+y+z)4
=2(x4+y4+z4)-(x2+y2+z2)2+(x+y+z)2[-2(x2+y2+z2)+(x+y+z)2]
tới đây r` sao đặt ẩn phân tích tiếp chắc =="
=2(x4+y4+z4+xy+xz+yz)