70. Phân tích các đa thức sau thành nhân tử :
a) x3 – 7x – 6 b) x3 – x2 – 14x + 24 c) x5 + x + 1
d) x8 + x7 + 1 e) x5 + x4 + 1 f) x10 + x5 + 1.
71. Phân tích các đa thức sau thành nhân tử :
a) (x + 2)(x + 3)(x + 4)(x + 5) – 24 b) (x + 1)(x + 2)(x + 4)(x + 5) – 40
c) x(x + 2)(x2 – 1) – 24 d) (2x + 1)(x + 1)2(2x + 3) – 18
e) (4x + 1)2(x + 1)(2x + 1) – 810 f) (12x + 7)2(3x + 2)(2x + 1) – 3.
72. Phân tích các đa thức sau thành nhân tử
a) (x2 – 2x + 4)(x2 + 3x + 4) – 14x2 b) (x + 2)(x + 4)2(x + 8) – 63x3
c) a2(b – c) + b2(c – a) + c2(a – b) d) a2b2(a – b) + b2c2(b – c) + c2a2(c – a).
e) a3 + b3 + c3 – 3abc.
\(x^3-7x-6\)
\(=x^3-3x^2+3x^2-9x+2x-6\)
\(=x^2\left(x-3\right)+3x\left(x-3\right)+2\left(x-3\right)\)
\(=\left(x-3\right)\left(x^2+3x+2\right)\)
\(=\left(x-3\right)\left(x+1\right)\left(x+2\right)\)
\(x^8+x^7+1\)
\(=\left(x^8-x^6+x^5-x^3+x^2\right)+\left(x^7-x^5+x^4-x^2+x\right)+\left(x^6-x^4+x^3-x+1\right)\)
\(=x^2\left(x^6-x^4+x^3-x+1\right)+x\left(x^6-x^4+x^3-x+1\right)+\left(x^6-x^4+x^3-x+1\right)\)
\(=\left(x^2+x+1\right)\left(x^6-x^4+x^3-x+1\right)\)