Solve x⁴ + x³ - 3x² - 5x - 2 = 0
The solutions of x⁴ + x³ - 3x² - 5x - 2 = 0 are x = -1, -1, -1, 2 (repeated roots counted once each: -1, 2).
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 -2, so the candidates are ±1, ±2.
- 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 - x - 2 = 0. Solve it (factoring or the quadratic formula):
- 5
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{4} + x^{3} - 3x^{2} - 5x - 2 = (x + 1)^{3}(x - 2) = 0 \]
\[ x^{4} + x^{3} - 3x^{2} - 5x - 2 = 0 \]
\[ \begin{array}{c|ccccc} -1 & 1 & 1 & -3 & -5 & -2 \\ & 0 & -1 & 0 & 3 & 2 \\ \hline & 1 & 0 & -3 & -2 & 0 \end{array} \]
\[ \begin{array}{c|cccc} -1 & 1 & 0 & -3 & -2 \\ & 0 & -1 & 1 & 2 \\ \hline & 1 & -1 & -2 & 0 \end{array} \]
\[ x^{2} - x - 2 = 0 \Rightarrow x = 2,\ -1 \]
\[ x^{4} + x^{3} - 3x^{2} - 5x - 2 = (x + 1)^{3}(x - 2) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -8 | no |
| -1 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | -1 | -1 |
| Product of the roots | -2 | -2 |
Questions
What are the roots of x⁴ + x³ - 3x² - 5x - 2 = 0?
The solutions of x⁴ + x³ - 3x² - 5x - 2 = 0 are x = -1, -1, -1, 2 (repeated roots counted once each: -1, 2).
How do you factor x⁴ + x³ - 3x² - 5x - 2?
(x + 1)(x − 2).
What is the sum of the roots?
-1 (it equals −1 by Vieta's formulas).