Solve x⁴ - 10x³ + 23x² + 34x - 120 = 0
The solutions of x⁴ - 10x³ + 23x² + 34x - 120 = 0 are x = -2, 3, 4, 5.
How to work it out
- 1
A quartic has up to 4 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 -120, so the candidates are ±1, ±2, ±3, ±4, ±5, ±6, ±8, ±10, ±12, ±15, ±20, ±24, ±30, ±40, ±60, ±120.
- 2
Test the candidates until one gives 0. x = -2 is a root. Divide by (x + 2) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 3
x = 3 is a root. Divide by (x − 3) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 4
What is left is the quadratic x² - 9x + 20 = 0. Solve it (factoring or the quadratic formula):
- 5
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{4} - 10x^{3} + 23x^{2} + 34x - 120 = (x + 2)(x - 3)(x - 4)(x - 5) = 0 \]
\[ x^{4} - 10x^{3} + 23x^{2} + 34x - 120 = 0 \]
\[ \begin{array}{c|ccccc} -2 & 1 & -10 & 23 & 34 & -120 \\ & 0 & -2 & 24 & -94 & 120 \\ \hline & 1 & -12 & 47 & -60 & 0 \end{array} \]
\[ \begin{array}{c|cccc} 3 & 1 & -12 & 47 & -60 \\ & 0 & 3 & -27 & 60 \\ \hline & 1 & -9 & 20 & 0 \end{array} \]
\[ x^{2} - 9x + 20 = 0 \Rightarrow x = 5,\ 4 \]
\[ x^{4} - 10x^{3} + 23x^{2} + 34x - 120 = (x + 2)(x - 3)(x - 4)(x - 5) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -72 | no |
| -1 | -120 | no |
| 2 | -24 | no |
| -2 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | 10 | 10 |
| Product of the roots | -120 | -120 |
Questions
What are the roots of x⁴ - 10x³ + 23x² + 34x - 120 = 0?
The solutions of x⁴ - 10x³ + 23x² + 34x - 120 = 0 are x = -2, 3, 4, 5.
How do you factor x⁴ - 10x³ + 23x² + 34x - 120?
(x + 2)(x − 3)(x − 4)(x − 5).
What is the sum of the roots?
10 (it equals −(-10) by Vieta's formulas).