Solve x³ - 7x² + 7x + 15 = 0
The solutions of x³ - 7x² + 7x + 15 = 0 are x = -1, 3, 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 15, so the candidates are ±1, ±3, ±5, ±15.
- 2
Test the candidates until one gives 0. x = -1 is a root. Divide by (x + 1) 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
\[ x^{3} - 7x^{2} + 7x + 15 = (x + 1)(x - 3)(x - 5) = 0 \]
\[ x^{3} - 7x^{2} + 7x + 15 = 0 \]
\[ \begin{array}{c|cccc} -1 & 1 & -7 & 7 & 15 \\ & 0 & -1 & 8 & -15 \\ \hline & 1 & -8 & 15 & 0 \end{array} \]
\[ x^{2} - 8x + 15 = 0 \Rightarrow x = 5,\ 3 \]
\[ x^{3} - 7x^{2} + 7x + 15 = (x + 1)(x - 3)(x - 5) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | 16 | no |
| -1 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | 7 | 7 |
| Product of the roots | -15 | -15 |
Questions
What are the roots of x³ - 7x² + 7x + 15 = 0?
The solutions of x³ - 7x² + 7x + 15 = 0 are x = -1, 3, 5.
How do you factor x³ - 7x² + 7x + 15?
(x + 1)(x − 3)(x − 5).
What is the sum of the roots?
7 (it equals −(-7) by Vieta's formulas).