Solve x³ + 8x² + 15x = 0
The solutions of x³ + 8x² + 15x = 0 are x = -5, -3, 0.
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 0 (it is 0, so x = 0 is a root and x factors out).
- 2
Test the candidates until one gives 0. x = 0 is a root. Divide by (x − 0) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 3
What is left is the quadratic x² + 8x + 15 = 0. Solve it (factoring or the quadratic formula):
- 4
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{3} + 8x^{2} + 15x = (x + 5)(x + 3)x = 0 \]
\[ x^{3} + 8x^{2} + 15x = 0 \]
\[ \begin{array}{c|cccc} 0 & 1 & 8 & 15 & 0 \\ & 0 & 0 & 0 & 0 \\ \hline & 1 & 8 & 15 & 0 \end{array} \]
\[ x^{2} + 8x + 15 = 0 \Rightarrow x = -3,\ -5 \]
\[ x^{3} + 8x^{2} + 15x = (x + 5)(x + 3)x \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 0 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | -8 | -8 |
| Product of the roots | 0 | 0 |
Questions
What are the roots of x³ + 8x² + 15x = 0?
The solutions of x³ + 8x² + 15x = 0 are x = -5, -3, 0.
How do you factor x³ + 8x² + 15x?
(x + 5)(x + 3)x.
What is the sum of the roots?
-8 (it equals −8 by Vieta's formulas).