Solve x⁴ - 4x³ - 2x² + 12x + 9 = 0
The solutions of x⁴ - 4x³ - 2x² + 12x + 9 = 0 are x = -1, -1, 3, 3 (repeated roots counted once each: -1, 3).
How to work it out
- 1
A quartic has up to 4 real roots. The rational root theorem says that any integer root of a polynomial with integer coefficients and leading coefficient 1 divides the constant term 9, so the candidates are ±1, ±3, ±9.
- 2
Test the candidates until one gives 0. x = -1 is a root. Divide by (x + 1) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 3
x = -1 is a root. Divide by (x + 1) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 4
What is left is the quadratic x² - 6x + 9 = 0. Solve it (factoring or the quadratic formula):
- 5
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{4} - 4x^{3} - 2x^{2} + 12x + 9 = (x + 1)^{2}(x - 3)^{2} = 0 \]
\[ x^{4} - 4x^{3} - 2x^{2} + 12x + 9 = 0 \]
\[ \begin{array}{c|ccccc} -1 & 1 & -4 & -2 & 12 & 9 \\ & 0 & -1 & 5 & -3 & -9 \\ \hline & 1 & -5 & 3 & 9 & 0 \end{array} \]
\[ \begin{array}{c|cccc} -1 & 1 & -5 & 3 & 9 \\ & 0 & -1 & 6 & -9 \\ \hline & 1 & -6 & 9 & 0 \end{array} \]
\[ x^{2} - 6x + 9 = 0 \Rightarrow x = 3 \]
\[ x^{4} - 4x^{3} - 2x^{2} + 12x + 9 = (x + 1)^{2}(x - 3)^{2} \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | 16 | no |
| -1 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | 4 | 4 |
| Product of the roots | 9 | 9 |
Questions
What are the roots of x⁴ - 4x³ - 2x² + 12x + 9 = 0?
The solutions of x⁴ - 4x³ - 2x² + 12x + 9 = 0 are x = -1, -1, 3, 3 (repeated roots counted once each: -1, 3).
How do you factor x⁴ - 4x³ - 2x² + 12x + 9?
(x + 1)(x − 3).
What is the sum of the roots?
4 (it equals −(-4) by Vieta's formulas).