Solve x⁴ + 4x³ - 11x² - 66x - 72 = 0
The solutions of x⁴ + 4x³ - 11x² - 66x - 72 = 0 are x = -3, -3, -2, 4 (repeated roots counted once each: -3, -2, 4).
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 -72, so the candidates are ±1, ±2, ±3, ±4, ±6, ±8, ±9, ±12, ±18, ±24, ±36, ±72.
- 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 = -3 is a root. Divide by (x + 3) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 4
What is left is the quadratic x² - x - 12 = 0. Solve it (factoring or the quadratic formula):
- 5
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{4} + 4x^{3} - 11x^{2} - 66x - 72 = (x + 3)^{2}(x + 2)(x - 4) = 0 \]
\[ x^{4} + 4x^{3} - 11x^{2} - 66x - 72 = 0 \]
\[ \begin{array}{c|ccccc} -2 & 1 & 4 & -11 & -66 & -72 \\ & 0 & -2 & -4 & 30 & 72 \\ \hline & 1 & 2 & -15 & -36 & 0 \end{array} \]
\[ \begin{array}{c|cccc} -3 & 1 & 2 & -15 & -36 \\ & 0 & -3 & 3 & 36 \\ \hline & 1 & -1 & -12 & 0 \end{array} \]
\[ x^{2} - x - 12 = 0 \Rightarrow x = 4,\ -3 \]
\[ x^{4} + 4x^{3} - 11x^{2} - 66x - 72 = (x + 3)^{2}(x + 2)(x - 4) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -144 | no |
| -1 | -20 | no |
| 2 | -200 | no |
| -2 | 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 | -72 | -72 |
Questions
What are the roots of x⁴ + 4x³ - 11x² - 66x - 72 = 0?
The solutions of x⁴ + 4x³ - 11x² - 66x - 72 = 0 are x = -3, -3, -2, 4 (repeated roots counted once each: -3, -2, 4).
How do you factor x⁴ + 4x³ - 11x² - 66x - 72?
(x + 3)(x + 2)(x − 4).
What is the sum of the roots?
-4 (it equals −4 by Vieta's formulas).