Phân tích đa thức thành nhân tử
\(x^{12}+x^6+1\)
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\(x^{12}-3x^6+1=\left(x^{12}+x^9-x^6\right)-\left(x^9-x^3+x^6\right)-\left(x^3-1+x^6\right)=x^6\left(x^6+x^3-1\right)-x^3\left(x^6+x^3-1\right)-\left(x^6+x^3-1\right)\)
\(=\left(x^6+x^3-1\right)\left(x^6-x^3-1\right)\)
\(4\left(x+5\right)\left(x+6\right)\left(x+10\right)\left(x+12\right)-3x^2\)
\(=4\left[\left(x+5\right)\left(x+12\right)\right]\left[\left(x+6\right)\left(x+10\right)\right]-3x^2\)
\(=4\left(x^2+17x+60\right)\left(x^2+16x+60\right)-3x^2\)
\(=\left(2x^2+34x+120\right)\left(2x^2+32x+60\right)-3x^2\)
\(=\left(2x^2+33x+120\right)^2-x^2-3x^2\)
\(=\left(2x^2+33x+120-2x\right)\left(2x^2+33x+120+2x\right)\)
\(=\left(2x+15\right)\left(x+8\right)\left(2x^2+35x+120\right)\)
Ta có: \(\left(x^2+x+1\right)\left(x^2+x+2\right)-12\)
\(=\left(x^2+x\right)^2+3\left(x^2+x\right)+2-12\)
\(=\left(x^2+x\right)^2+3\left(x^2+x\right)-10\)
\(=\left(x^2+x+5\right)\left(x^2+x-2\right)\)
\(=\left(x^2+x+5\right)\left(x+2\right)\left(x-1\right)\)
\(4\left(x+5\right)\left(x+12\right)\left(x+6\right)\left(x+10\right)-3x^2\)
\(=2\left(x^2+60+17x\right).2\left(x^2+60+16x\right)-3x^2\)
\(=\left(2x^2+120+33x+x\right)\left(2x^2+120+33x-x\right)-3x^2\)
\(=\left(2x^2+120+33x\right)^2-x^2-3x^2\)
\(=\left(2x^2+120+33x\right)^2-4x^2\)
\(=\left(2x^2+120+33x+2x\right)\left(2x^2+120+33x-2x\right)\)
\(=\left(2x^2+35x+120\right)\left(2x^2+31x+120\right)\)
\(=\left(2x^2+35x+120\right)\left(x+8\right)\left(2x+15\right)\)
(x2+x+1)(x2+x+2)-12
=(x2+x+1)[(x2+x+1)+1)-12
=(x2+x+1)2+(x2+x+1)-12
=(x2+x+1)2-3.(x2+x+1)+4.(x2+x+1)-12
=(x2+x+1)(x2+x+1-3)+4.(x2+x+1-3)
=(x2+x+1)(x2+x-2)+4.(x2+x-2)
=(x2+x-2)(x2+x+1+4)
=(x2-x+2x-2)(x2+x+5)
=[x.(x-1)+2.(x-1)](x2+x+5)
=(x-1)(x+2)(x2+x+5)
(x^2+x+1)(x^2+x+2)-12
Đặt x^2+x+1= a ta có
=a^2+a-12
=a^2-3a+4a-12
=(a^2-3a)+(4a-12)
=a(a-3)+4(a-3)
=(a-3)(a+4)
thay x^2+x+1=a ta được
(x^2+x-2)(x^2+x+5)
#)Giải :
\(x^3-2x-4\)
\(=x^3+2x^2-2x^2+2x-4x-4\)
\(=x^3+2x^2+2x-2x^2-4x-4\)
\(=x\left(x^2+2x+2\right)-2\left(x^2+2x+2\right)\)
\(=\left(x-2\right)\left(x^2+2x+2\right)\)
\(x^4+2x^3+5x^2+4x-12\)
\(=x^4+x^3+6x^2+x^3+x^2+6x-2x^2-2x-12\)
\(=x^2\left(x^2+x+6\right)+x\left(x^2+x+6\right)-2\left(x^2+x+6\right)\)
\(=\left(x^2+x+6\right)\left(x^2+x-2\right)\)
\(=\left(x^2+x+6\right)\left(x-1\right)\left(x+2\right)\)
Câu 1.
Đoán được nghiệm là 2.Ta giải như sau:
\(x^3-2x-4\)
\(=x^3-2x^2+2x^2-4x+2x-4\)
\(=x^2\left(x-2\right)+2x\left(x-2\right)+2\left(x-2\right)\)
\(=\left(x-2\right)\left(x^2+2x+2\right)\)
= X12 +2x6 +1 - x6
= ( x6 +1)2 - x6
= ( x6 - x3 +1 )( x6 + x3 +1 )