Solve x⁴ + 8x³ + 11x² - 32x - 60 = 0
The solutions of x⁴ + 8x³ + 11x² - 32x - 60 = 0 are x = -5, -3, -2, 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 -60, so the candidates are ±1, ±2, ±3, ±4, ±5, ±6, ±10, ±12, ±15, ±20, ±30, ±60.
- 2
Test the candidates until one gives 0. x = 2 is a root. Divide by (x − 2) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 3
x = -2 is a root. Divide by (x + 2) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 4
What is left is the quadratic x² + 8x + 15 = 0. Solve it (factoring or the quadratic formula):
- 5
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{4} + 8x^{3} + 11x^{2} - 32x - 60 = (x + 5)(x + 3)(x + 2)(x - 2) = 0 \]
\[ x^{4} + 8x^{3} + 11x^{2} - 32x - 60 = 0 \]
\[ \begin{array}{c|ccccc} 2 & 1 & 8 & 11 & -32 & -60 \\ & 0 & 2 & 20 & 62 & 60 \\ \hline & 1 & 10 & 31 & 30 & 0 \end{array} \]
\[ \begin{array}{c|cccc} -2 & 1 & 10 & 31 & 30 \\ & 0 & -2 & -16 & -30 \\ \hline & 1 & 8 & 15 & 0 \end{array} \]
\[ x^{2} + 8x + 15 = 0 \Rightarrow x = -3,\ -5 \]
\[ x^{4} + 8x^{3} + 11x^{2} - 32x - 60 = (x + 5)(x + 3)(x + 2)(x - 2) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -72 | no |
| -1 | -24 | no |
| 2 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | -8 | -8 |
| Product of the roots | -60 | -60 |
Questions
What are the roots of x⁴ + 8x³ + 11x² - 32x - 60 = 0?
The solutions of x⁴ + 8x³ + 11x² - 32x - 60 = 0 are x = -5, -3, -2, 2.
How do you factor x⁴ + 8x³ + 11x² - 32x - 60?
(x + 5)(x + 3)(x + 2)(x − 2).
What is the sum of the roots?
-8 (it equals −8 by Vieta's formulas).