Solve x³ - 15x² + 75x - 125 = 0
The solutions of x³ - 15x² + 75x - 125 = 0 are x = 5, 5, 5 (repeated roots counted once each: 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 -125, so the candidates are ±1, ±5, ±25, ±125.
- 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 - 10x + 25 = 0. Solve it (factoring or the quadratic formula):
- 4
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{3} - 15x^{2} + 75x - 125 = (x - 5)^{3} = 0 \]
\[ x^{3} - 15x^{2} + 75x - 125 = 0 \]
\[ \begin{array}{c|cccc} 5 & 1 & -15 & 75 & -125 \\ & 0 & 5 & -50 & 125 \\ \hline & 1 & -10 & 25 & 0 \end{array} \]
\[ x^{2} - 10x + 25 = 0 \Rightarrow x = 5 \]
\[ x^{3} - 15x^{2} + 75x - 125 = (x - 5)^{3} \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -64 | no |
| -1 | -216 | no |
| 5 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | 15 | 15 |
| Product of the roots | 125 | 125 |
Questions
What are the roots of x³ - 15x² + 75x - 125 = 0?
The solutions of x³ - 15x² + 75x - 125 = 0 are x = 5, 5, 5 (repeated roots counted once each: 5).
How do you factor x³ - 15x² + 75x - 125?
(x − 5).
What is the sum of the roots?
15 (it equals −(-15) by Vieta's formulas).