Solve x³ + 5x² - 16x - 80 = 0
The solutions of x³ + 5x² - 16x - 80 = 0 are x = -5, -4, 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 -80, so the candidates are ±1, ±2, ±4, ±5, ±8, ±10, ±16, ±20, ±40, ±80.
- 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 + 9x + 20 = 0. Solve it (factoring or the quadratic formula):
- 4
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{3} + 5x^{2} - 16x - 80 = (x + 5)(x + 4)(x - 4) = 0 \]
\[ x^{3} + 5x^{2} - 16x - 80 = 0 \]
\[ \begin{array}{c|cccc} 4 & 1 & 5 & -16 & -80 \\ & 0 & 4 & 36 & 80 \\ \hline & 1 & 9 & 20 & 0 \end{array} \]
\[ x^{2} + 9x + 20 = 0 \Rightarrow x = -4,\ -5 \]
\[ x^{3} + 5x^{2} - 16x - 80 = (x + 5)(x + 4)(x - 4) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -90 | no |
| -1 | -60 | no |
| 2 | -84 | no |
| -2 | -36 | no |
| 4 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | -5 | -5 |
| Product of the roots | 80 | 80 |
Questions
What are the roots of x³ + 5x² - 16x - 80 = 0?
The solutions of x³ + 5x² - 16x - 80 = 0 are x = -5, -4, 4.
How do you factor x³ + 5x² - 16x - 80?
(x + 5)(x + 4)(x − 4).
What is the sum of the roots?
-5 (it equals −5 by Vieta's formulas).