Solve x³ - 2x² - 15x + 36 = 0
The solutions of x³ - 2x² - 15x + 36 = 0 are x = -4, 3, 3 (repeated roots counted once each: -4, 3).
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 36, so the candidates are ±1, ±2, ±3, ±4, ±6, ±9, ±12, ±18, ±36.
- 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 + x - 12 = 0. Solve it (factoring or the quadratic formula):
- 4
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{3} - 2x^{2} - 15x + 36 = (x + 4)(x - 3)^{2} = 0 \]
\[ x^{3} - 2x^{2} - 15x + 36 = 0 \]
\[ \begin{array}{c|cccc} 3 & 1 & -2 & -15 & 36 \\ & 0 & 3 & 3 & -36 \\ \hline & 1 & 1 & -12 & 0 \end{array} \]
\[ x^{2} + x - 12 = 0 \Rightarrow x = 3,\ -4 \]
\[ x^{3} - 2x^{2} - 15x + 36 = (x + 4)(x - 3)^{2} \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | 20 | no |
| -1 | 48 | no |
| 2 | 6 | no |
| -2 | 50 | no |
| 3 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | 2 | 2 |
| Product of the roots | -36 | -36 |
Questions
What are the roots of x³ - 2x² - 15x + 36 = 0?
The solutions of x³ - 2x² - 15x + 36 = 0 are x = -4, 3, 3 (repeated roots counted once each: -4, 3).
How do you factor x³ - 2x² - 15x + 36?
(x + 4)(x − 3).
What is the sum of the roots?
2 (it equals −(-2) by Vieta's formulas).