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#)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:
\(x^3-2x-4\)
\(=x^3-2x^2+2x^2-4x+2x-4\)
\(=x^2\left(x-2\right)+2x\left(x-2\right)+2\left(x-2\right)\)
\(=\left(x-2\right)\left(x^2+2x+2\right)\)
Ta có: \(\left(x^2+x+1\right)\left(x^2+x+2\right)-12\)
\(=\left(x^2+x\right)^2+3\left(x^2+x\right)+2-12\)
\(=\left(x^2+x\right)^2+3\left(x^2+x\right)-10\)
\(=\left(x^2+x+5\right)\left(x^2+x-2\right)\)
\(=\left(x^2+x+5\right)\left(x+2\right)\left(x-1\right)\)
\(x^2-x-12=x^2-3x+4x-12\)
\(=\left(x^2-3x\right)+\left(4x-12\right)\)
\(=x\left(x-3\right)+4\left(x-3\right)\)
\(=\left(x-3\right)\left(x+4\right)\)
\(x^2-x-12=x^2-4x+3x-12\)
\(=x\left(x-4\right)+3\left(x-4\right)\)
\(=\left(x-4\right)\left(x+3\right)\)
\(=\left(x^2+x\right)^2+4\left(x^2+x\right)+4-16\\ =\left(x^2+x+2\right)^2-16\\ =\left(x^2+x+2-4\right)\left(x^2+x+2+4\right)\\ =\left(x^2+x-2\right)\left(x^2+x+6\right)\\ =\left(x-1\right)\left(x+2\right)\left(x^2+x+6\right)\)
=\(x^4+2x^3+x^2+4x^2+4x-12\)
=\(x^4+2x^3+5x^2+4x-12\)
=\(x^4-x^3+3x^3-3x^2+8x^2+4x-12\)
=\(x^3(x-1)+3x^2(x-1)+4(2x^2+x-3)\)
=\(x^3(x-1)+3x^2(x-1)+4(2x^2-2x+3x-3)\)
=\(x^3(x-1)+3x^2(x-1)+4[2x(x-1)+3(x-1)]\)
=\(x^3(x-1)+3x^2(x-1)+4(x-1)(2x+3)\)
=\((x-1)[x^3+3x^2+4(2x+3)]\)
=\((x-1)(x^3+3x^2+8x+12)\)
\(\left(x^2+x\right)^2+\left(4x^2+4x\right)+4-16\\ =\left(x^2+x+2\right)^2-16\\ =\left(x^2+x+2-4\right)\left(x^2+x+2+4\right)\\ =\left(x^2+x-2\right)\left(x^2+x+6\right)\)
\(x^2+x-2\) vẫn còn phân tích được nữa bạn nhé.
\(x^2+x-2=\left(x-1\right)\left(x+2\right)\)
\(x^2-8x+12=\left(x^2-6x\right)-\left(2x-12\right)=x\left(x-6\right)-2\left(x-6\right)=\left(x-2\right)\left(x-6\right)\)
(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)
\(x^2-x-12\\ =x^2-4x+3x-12\\ =x\left(x-4\right)+3\left(x-4\right)\\ =\left(x-4\right)\left(x+3\right)\)