(x - 3)3 - (x + 1)3 + 12x (x - 1)
= x3 - 3x2 . 3 + 3x . 32 - 27 - (x3 + 3x2 . 1 + 3x . 12 + 13) + 12x . x + 12x . (-1)
= x3 - 9x2 + 27x - 27 - x3 - 3x2 - 3x - 1 + 12x2 - 12x
= (x3 - x3) + (12x2 - 9x2 - 3x2) + (27x - 3x - 12x) - (27 + 1)
= 12x - 28
\(\left(x-3\right)^3-\left(x+1\right)^3+12x\left(x-1\right)\)
\(\Leftrightarrow\left(x^3-3x^23+3x3^2-3^3\right)-\left(x^3+3x^21+3x1^2+1^3\right)+12x^2-12x\)
\(\Leftrightarrow x^3-9x^2+27x-27-x^3-3x^2-3x-1+12x^2-12x\)
\(\Leftrightarrow12x-28=0\)
\(\Leftrightarrow12x=28\)
\(\Leftrightarrow x=\frac{7}{3}\)
Vậy S={\(\frac{7}{3}\)} là nghiệm pt
