Solve x³ + 5x² - 36x - 180 = 0
The solutions of x³ + 5x² - 36x - 180 = 0 are x = -6, -5, 6.
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 -180, so the candidates are ±1, ±2, ±3, ±4, ±5, ±6, ±9, ±10, ±12, ±15, ±18, ±20, ±30, ±36, ±45, ±60, ±90, ±180.
- 2
Test the candidates until one gives 0. x = -5 is a root. Divide by (x + 5) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 3
What is left is the quadratic x² - 36 = 0. Solve it (factoring or the quadratic formula):
- 4
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{3} + 5x^{2} - 36x - 180 = (x + 6)(x + 5)(x - 6) = 0 \]
\[ x^{3} + 5x^{2} - 36x - 180 = 0 \]
\[ \begin{array}{c|cccc} -5 & 1 & 5 & -36 & -180 \\ & 0 & -5 & 0 & 180 \\ \hline & 1 & 0 & -36 & 0 \end{array} \]
\[ x^{2} - 36 = 0 \Rightarrow x = 6,\ -6 \]
\[ x^{3} + 5x^{2} - 36x - 180 = (x + 6)(x + 5)(x - 6) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -210 | no |
| -1 | -140 | no |
| 2 | -224 | no |
| -2 | -96 | no |
| 3 | -216 | no |
| -3 | -54 | no |
| 4 | -180 | no |
| -4 | -20 | no |
| 5 | -110 | no |
| -5 | 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 | 180 | 180 |
Questions
What are the roots of x³ + 5x² - 36x - 180 = 0?
The solutions of x³ + 5x² - 36x - 180 = 0 are x = -6, -5, 6.
How do you factor x³ + 5x² - 36x - 180?
(x + 6)(x + 5)(x − 6).
What is the sum of the roots?
-5 (it equals −5 by Vieta's formulas).