Solve x³ - 5x² - 4x + 20 = 0
The solutions of x³ - 5x² - 4x + 20 = 0 are x = -2, 2, 5.
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 20, so the candidates are ±1, ±2, ±4, ±5, ±10, ±20.
- 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
What is left is the quadratic x² - 3x - 10 = 0. Solve it (factoring or the quadratic formula):
- 4
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{3} - 5x^{2} - 4x + 20 = (x + 2)(x - 2)(x - 5) = 0 \]
\[ x^{3} - 5x^{2} - 4x + 20 = 0 \]
\[ \begin{array}{c|cccc} 2 & 1 & -5 & -4 & 20 \\ & 0 & 2 & -6 & -20 \\ \hline & 1 & -3 & -10 & 0 \end{array} \]
\[ x^{2} - 3x - 10 = 0 \Rightarrow x = 5,\ -2 \]
\[ x^{3} - 5x^{2} - 4x + 20 = (x + 2)(x - 2)(x - 5) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | 12 | no |
| -1 | 18 | no |
| 2 | 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 | -20 | -20 |
Questions
What are the roots of x³ - 5x² - 4x + 20 = 0?
The solutions of x³ - 5x² - 4x + 20 = 0 are x = -2, 2, 5.
How do you factor x³ - 5x² - 4x + 20?
(x + 2)(x − 2)(x − 5).
What is the sum of the roots?
5 (it equals −(-5) by Vieta's formulas).