Solve x³ - 3x² - 9x - 5 = 0
The solutions of x³ - 3x² - 9x - 5 = 0 are x = -1, -1, 5 (repeated roots counted once each: -1, 5).
How to work it out
- 1
A cubic has up to 3 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 -5, so the candidates are ±1, ±5.
- 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
What is left is the quadratic x² - 4x - 5 = 0. Solve it (factoring or the quadratic formula):
- 4
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{3} - 3x^{2} - 9x - 5 = (x + 1)^{2}(x - 5) = 0 \]
\[ x^{3} - 3x^{2} - 9x - 5 = 0 \]
\[ \begin{array}{c|cccc} -1 & 1 & -3 & -9 & -5 \\ & 0 & -1 & 4 & 5 \\ \hline & 1 & -4 & -5 & 0 \end{array} \]
\[ x^{2} - 4x - 5 = 0 \Rightarrow x = 5,\ -1 \]
\[ x^{3} - 3x^{2} - 9x - 5 = (x + 1)^{2}(x - 5) \]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 | 3 | 3 |
| Product of the roots | 5 | 5 |
Questions
What are the roots of x³ - 3x² - 9x - 5 = 0?
The solutions of x³ - 3x² - 9x - 5 = 0 are x = -1, -1, 5 (repeated roots counted once each: -1, 5).
How do you factor x³ - 3x² - 9x - 5?
(x + 1)(x − 5).
What is the sum of the roots?
3 (it equals −(-3) by Vieta's formulas).