Solve x³ - 6x² - 15x + 100 = 0
The solutions of x³ - 6x² - 15x + 100 = 0 are x = -4, 5, 5 (repeated roots counted once each: -4, 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 100, so the candidates are ±1, ±2, ±4, ±5, ±10, ±20, ±25, ±50, ±100.
- 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 - 10x + 25 = 0. Solve it (factoring or the quadratic formula):
- 4
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{3} - 6x^{2} - 15x + 100 = (x + 4)(x - 5)^{2} = 0 \]
\[ x^{3} - 6x^{2} - 15x + 100 = 0 \]
\[ \begin{array}{c|cccc} -4 & 1 & -6 & -15 & 100 \\ & 0 & -4 & 40 & -100 \\ \hline & 1 & -10 & 25 & 0 \end{array} \]
\[ x^{2} - 10x + 25 = 0 \Rightarrow x = 5 \]
\[ x^{3} - 6x^{2} - 15x + 100 = (x + 4)(x - 5)^{2} \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | 80 | no |
| -1 | 108 | no |
| 2 | 54 | no |
| -2 | 98 | no |
| 4 | 8 | no |
| -4 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | 6 | 6 |
| Product of the roots | -100 | -100 |
Questions
What are the roots of x³ - 6x² - 15x + 100 = 0?
The solutions of x³ - 6x² - 15x + 100 = 0 are x = -4, 5, 5 (repeated roots counted once each: -4, 5).
How do you factor x³ - 6x² - 15x + 100?
(x + 4)(x − 5).
What is the sum of the roots?
6 (it equals −(-6) by Vieta's formulas).