Solve x⁴ + 13x³ + 61x² + 123x + 90 = 0
The solutions of x⁴ + 13x³ + 61x² + 123x + 90 = 0 are x = -5, -3, -3, -2 (repeated roots counted once each: -5, -3, -2).
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 90, so the candidates are ±1, ±2, ±3, ±5, ±6, ±9, ±10, ±15, ±18, ±30, ±45, ±90.
- 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 + 8x + 15 = 0. Solve it (factoring or the quadratic formula):
- 5
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{4} + 13x^{3} + 61x^{2} + 123x + 90 = (x + 5)(x + 3)^{2}(x + 2) = 0 \]
\[ x^{4} + 13x^{3} + 61x^{2} + 123x + 90 = 0 \]
\[ \begin{array}{c|ccccc} -2 & 1 & 13 & 61 & 123 & 90 \\ & 0 & -2 & -22 & -78 & -90 \\ \hline & 1 & 11 & 39 & 45 & 0 \end{array} \]
\[ \begin{array}{c|cccc} -3 & 1 & 11 & 39 & 45 \\ & 0 & -3 & -24 & -45 \\ \hline & 1 & 8 & 15 & 0 \end{array} \]
\[ x^{2} + 8x + 15 = 0 \Rightarrow x = -3,\ -5 \]
\[ x^{4} + 13x^{3} + 61x^{2} + 123x + 90 = (x + 5)(x + 3)^{2}(x + 2) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | 288 | no |
| -1 | 16 | no |
| 2 | 700 | no |
| -2 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | -13 | -13 |
| Product of the roots | 90 | 90 |
Questions
What are the roots of x⁴ + 13x³ + 61x² + 123x + 90 = 0?
The solutions of x⁴ + 13x³ + 61x² + 123x + 90 = 0 are x = -5, -3, -3, -2 (repeated roots counted once each: -5, -3, -2).
How do you factor x⁴ + 13x³ + 61x² + 123x + 90?
(x + 5)(x + 3)(x + 2).
What is the sum of the roots?
-13 (it equals −13 by Vieta's formulas).