Solve x³ - 3x² - 36x + 108 = 0
The solutions of x³ - 3x² - 36x + 108 = 0 are x = -6, 3, 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 108, so the candidates are ±1, ±2, ±3, ±4, ±6, ±9, ±12, ±18, ±27, ±36, ±54, ±108.
- 2
Test the candidates until one gives 0. x = 3 is a root. Divide by (x − 3) 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
\[ x^{3} - 3x^{2} - 36x + 108 = (x + 6)(x - 3)(x - 6) = 0 \]
\[ x^{3} - 3x^{2} - 36x + 108 = 0 \]
\[ \begin{array}{c|cccc} 3 & 1 & -3 & -36 & 108 \\ & 0 & 3 & 0 & -108 \\ \hline & 1 & 0 & -36 & 0 \end{array} \]
\[ x^{2} - 36 = 0 \Rightarrow x = 6,\ -6 \]
\[ x^{3} - 3x^{2} - 36x + 108 = (x + 6)(x - 3)(x - 6) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | 70 | no |
| -1 | 140 | no |
| 2 | 32 | no |
| -2 | 160 | no |
| 3 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | 3 | 3 |
| Product of the roots | -108 | -108 |
Questions
What are the roots of x³ - 3x² - 36x + 108 = 0?
The solutions of x³ - 3x² - 36x + 108 = 0 are x = -6, 3, 6.
How do you factor x³ - 3x² - 36x + 108?
(x + 6)(x − 3)(x − 6).
What is the sum of the roots?
3 (it equals −(-3) by Vieta's formulas).