Đáp án `+` Giải thích các bước giải:
` x^4 - x^3 - 6x^2 + 15x - 9 `
`= x^4 - x^3 - 6x^2 + 6x + 9x - 9`
`= (x^4 - 6x^2 + 9x) - (x^3 - 6x + 9) `
`= x(x^3 - 6x + 9) - (x^3 - 6x + 9) `
`= (x^3 - 6x + 9)(x - 1) `
`= (x^3 + 3x^2 - 3x^2 - 9x + 3x + 9)(x-1) `
`= [(x^3 + 3x^2 ) - (3x^2 + 9x ) + (3x+9)](x-1) `
`= [(x^2(x+3) - 3x(x + 3) + 3(x +3 )](x-1) `
`= (x + 3)(x^2 - 3x +3)(x-1) `
