Solve x³ + 7x² - 14x - 120 = 0
The solutions of x³ + 7x² - 14x - 120 = 0 are x = -6, -5, 4.
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 -120, so the candidates are ±1, ±2, ±3, ±4, ±5, ±6, ±8, ±10, ±12, ±15, ±20, ±24, ±30, ±40, ±60, ±120.
- 2
Test the candidates until one gives 0. x = 4 is a root. Divide by (x − 4) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 3
What is left is the quadratic x² + 11x + 30 = 0. Solve it (factoring or the quadratic formula):
- 4
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{3} + 7x^{2} - 14x - 120 = (x + 6)(x + 5)(x - 4) = 0 \]
\[ x^{3} + 7x^{2} - 14x - 120 = 0 \]
\[ \begin{array}{c|cccc} 4 & 1 & 7 & -14 & -120 \\ & 0 & 4 & 44 & 120 \\ \hline & 1 & 11 & 30 & 0 \end{array} \]
\[ x^{2} + 11x + 30 = 0 \Rightarrow x = -5,\ -6 \]
\[ x^{3} + 7x^{2} - 14x - 120 = (x + 6)(x + 5)(x - 4) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -126 | no |
| -1 | -100 | no |
| 2 | -112 | no |
| -2 | -72 | no |
| 3 | -72 | no |
| -3 | -42 | no |
| 4 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | -7 | -7 |
| Product of the roots | 120 | 120 |
Questions
What are the roots of x³ + 7x² - 14x - 120 = 0?
The solutions of x³ + 7x² - 14x - 120 = 0 are x = -6, -5, 4.
How do you factor x³ + 7x² - 14x - 120?
(x + 6)(x + 5)(x − 4).
What is the sum of the roots?
-7 (it equals −7 by Vieta's formulas).