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a, ( x2 + x )2 - 14 ( x2 + x ) + 24
= (x2 + x)2 - 2(x2 + x) -12(x2 + x) + 24
= (x2 + x).(x2 + x -2) - 12(x2 + x -2)
= (x2 + x -2).(x2 + x -12)
= (x2 + 2x - x - 2).(x2 + 4x - 3x - 12)
=[x.(x+2)-(x+2)].[x.(x+4)-3(x+4)]
= (x+2).(x-1).(x+4).(x-3)
= x4 + 2x3 - 13x2 - 14x + 24
b, ( x2 + x )2 + 4x2 + 4x - 12
= x4 + 2x3 + x2 + 4x2 + 4x -12
= x4 + 2x3 + 5x2 + 4x -12
c, x4 + 2x3 + 5x2 + 4x - 12
= x4 - x3 + 3x3 - 3x2 + 8x2 - 8x +12x -12
= x3(x-1) + 3x2(x-1) + 8x(x-1) + 12(x-1)
= (x-1) . (x3 + 3x2 + 8x +12)
= (x-1) . ( x3 +2x2 + x2 + 2x + 6x +12)
= (x-1). [x2(x+2) + x(x+2) + 6(x+2)]
= (x-1).(x+2).(x2 + x+ 6)
\(x^8+x^7+1\)
\(=x^8+x^7+x^6-x^6+x^5-x^5+x^4-x^4+x^3-x^3+x^2-x^2+x-xx+1\)
\(=\left(x^8-x^6+x^5-x^3+x^2\right)\)
\(+\left(x^7-x^5+x^4-x^2+x\right)\)
\(+\left(x^6-x^4+x^3-x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^6-x^4+x^3-x+1\right)\)
bÀI LÀM
a) x4+x3+2x2+x+1=(x4+x3+x2)+(x2+x+1)=x2(x2+x+1)+(x2+x+1)=(x2+x+1)(x2+1)
b)a3+b3+c3-3abc=a3+3ab(a+b)+b3+c3 -(3ab(a+b)+3abc)=(a+b)3+c3-3ab(a+b+c)
=(a+b+c)((a+b)2-(a+b)c+c2)-3ab(a+b+c)=(a+b+c)(a2+2ab+b2-ac-ab+c2-3ab)=(a+b+c)(a2+b2+c2-ab-ac-bc)
c)Đặt x-y=a;y-z=b;z-x=c
a+b+c=x-y-z+z-x=o
đưa về như bài b
d)nhóm 2 hạng tử đầu lại và 2hangj tử sau lại để 2 hạng tử sau ở trong ngoặc sau đó áp dụng hằng đẳng thức dề tính sau đó dặt nhân tử chung
e)x2(y-z)+y2(z-x)+z2(x-y)=x2(y-z)-y2((y-z)+(x-y))+z2(x-y)
=x2(y-z)-y2(y-z)-y2(x-y)+z2(x-y)=(y-z)(x2-y2)-(x-y)(y2-z2)=(y-z)(x2-2y2+xy+xz+yz)
a, - Đặt \(x^2+4x+8=a\) ta được :\(a^2+3xa+2x^2\)
\(=a^2+xa+2xa+2x^2\)
\(=a\left(a+x\right)+2x\left(a+x\right)\)
\(=\left(2x+a\right)\left(x+a\right)\)
- Thay lại x vào đa thức ta được :
\(\left(2x+x^2+4x+8\right)\left(x+x^2+4x+8\right)\)
\(=\left(x^2+6x+8\right)\left(x^2+5x+8\right)\)
b, - Đặt \(x^2+x+1=a\) ta được :\(a\left(a+1\right)-12\)
\(=a^2+a-12\)
\(=a^2+\frac{1}{2}.2.a+\frac{1}{4}-\frac{49}{4}\)
\(=\left(a+\frac{1}{2}\right)^2-\left(\frac{7}{2}\right)^2\)
\(=\left(a+\frac{1}{2}+\frac{7}{2}\right)\left(a+\frac{1}{2}-\frac{7}{2}\right)\)
\(=\left(a+4\right)\left(a-3\right)\)
- Thay lại x vào đa thức ta được :
\(\left(x^2+x+1+4\right)\left(x^2+x+1-3\right)\)
\(=\left(x^2+x+5\right)\left(x^2+x-2\right)\)
c, - Đặt \(x^2+8x+7=a\) ta được : \(a\left(a+8\right)+15\)
\(=a^2+8a+15\)
\(=a^2+3a+5a+15\)
\(=a\left(a+3\right)+5\left(a+3\right)\)
\(=\left(a+3\right)\left(a+5\right)\)
- Thay lại x vào đa thức ta được :
\(\left(x^2+8x+7+3\right)\left(x^2+8x+7+5\right)\)
\(=\left(x^2+8x+10\right)\left(x^2+8x+12\right)\)
d, Ta có : \(\left(x+2\right)\left(x+3\right)\left(x+4\right)\left(x+5\right)-24\)
\(=\left(x^2+2x+5x+10\right)\left(x^2+3x+4x+12\right)-24\)
\(=\left(x^2+7x+10\right)\left(x^2+7x+12\right)-24\)
- Đặt \(x^2+7x+10=a\) ta được : \(a\left(a+2\right)-24\)
\(=a^2+2a-24\)
\(=a^2-4a+6a-24\)
\(=a\left(a-4\right)+6\left(a-4\right)\)
\(=\left(a+6\right)\left(a-4\right)\)
- Thay lại x vào đa thức ta được :
\(\left(x^2+7x+10+6\right)\left(x^2+7x+10-4\right)\)
\(=\left(x^2+7x+16\right)\left(x^2+7x+6\right)\)
a) Ta có: \(\left(x^2+x\right)^2-14\left(x^2+x\right)+24\)(1)
Đặt \(a=x^2+x\)
(1)\(=a^2-14a+24\)
\(=a^2-12a-2a+24\)
\(=a\left(a-12\right)-2\left(a-12\right)\)
\(=\left(a-12\right)\left(a-2\right)\)
\(=\left(x^2+x-12\right)\left(x^2+x-2\right)\)
\(=\left(x^2+4x-3x-12\right)\left(x^2+2x-x-2\right)\)
\(=\left[x\left(x+4\right)-3\left(x+4\right)\right]\left[x\left(x+2\right)-\left(x+2\right)\right]\)
\(=\left(x+4\right)\left(x-3\right)\left(x+2\right)\left(x-1\right)\)
b) Ta có: \(\left(x^2+x\right)^2+4x^2+4x-12\)
\(=\left(x^2+x\right)^2+4\left(x^2+x\right)-12\)
\(=a^2+4a-12\)
\(=a^2+6a-2a-12\)
\(=a\left(a+6\right)-2\left(a+6\right)\)
\(=\left(a+6\right)\left(a-2\right)\)
\(=\left(x^2+x+6\right)\left(x^2+x-2\right)\)
\(=\left(x^2+x+6\right)\left(x^2+2x-x-2\right)\)
\(=\left(x^2+x+6\right)\left[x\left(x+2\right)-\left(x+2\right)\right]\)
\(=\left(x^2+x+6\right)\left(x+2\right)\left(x-1\right)\)
c) Ta có: \(x^4+2x^3+5x^2+4x-12\)
\(=x^4-x^3+3x^3-3x^2+8x^2-8x+12x-12\)
\(=x^3\left(x-1\right)+3x^2\left(x-1\right)+8x\left(x-1\right)+12\left(x-1\right)\)
\(=\left(x-1\right)\left(x^3+3x^2+8x+12\right)\)
\(=\left(x-1\right)\left(x^3+2x^2+x^2+2x+6x+12\right)\)
\(=\left(x-1\right)\left[x^2\left(x+2\right)+x\left(x+2\right)+6\left(x+2\right)\right]\)
\(=\left(x-1\right)\left(x+2\right)\left(x^2+x+6\right)\)
d) Ta có: \(\left(x+1\right)\left(x+2\right)\left(x+3\right)\left(x+4\right)+1\)
\(=\left(x^2+5x+4\right)\left(x^2+5x+6\right)+1\)(2)
Đặt \(x^2+5x=b\)
(2)\(=\left(b+4\right)\left(b+6\right)+1\)
\(=b^2+10b+24+1\)
\(=b^2+10b+25\)
\(=\left(b+5\right)^2\)
\(=\left(x^2+5x+5\right)^2\)
e) Ta có: \(\left(x+1\right)\left(x+3\right)\left(x+5\right)\left(x+7\right)+15\)
\(=\left(x^2+8x+7\right)\left(x^2+8x+15\right)+15\)(3)
Đặt \(c=x^2+8x\)
(3)\(=\left(c+7\right)\left(c+15\right)+15\)
\(=c^2+22c+105+15\)
\(=c^2+22c+120\)
\(=c^2+12c+10c+120\)
\(=c\left(c+12\right)+10\left(c+12\right)\)
\(=\left(c+12\right)\left(c+10\right)\)
\(=\left(x^2+8x+12\right)\left(x^2+8x+10\right)\)
\(=\left(x^2+6x+2x+12\right)\left(x^2+8x+10\right)\)
\(=\left[x\left(x+6\right)+2\left(x+6\right)\right]\left(x^2+8x+10\right)\)
\(=\left(x+6\right)\left(x+2\right)\left(x^2+8x+10\right)\)
