Solve x⁴ + 9x³ + 9x² - 85x - 150 = 0
The solutions of x⁴ + 9x³ + 9x² - 85x - 150 = 0 are x = -5, -5, -2, 3 (repeated roots counted once each: -5, -2, 3).
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 -150, so the candidates are ±1, ±2, ±3, ±5, ±6, ±10, ±15, ±25, ±30, ±50, ±75, ±150.
- 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 + 10x + 25 = 0. Solve it (factoring or the quadratic formula):
- 5
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{4} + 9x^{3} + 9x^{2} - 85x - 150 = (x + 5)^{2}(x + 2)(x - 3) = 0 \]
\[ x^{4} + 9x^{3} + 9x^{2} - 85x - 150 = 0 \]
\[ \begin{array}{c|ccccc} -2 & 1 & 9 & 9 & -85 & -150 \\ & 0 & -2 & -14 & 10 & 150 \\ \hline & 1 & 7 & -5 & -75 & 0 \end{array} \]
\[ \begin{array}{c|cccc} 3 & 1 & 7 & -5 & -75 \\ & 0 & 3 & 30 & 75 \\ \hline & 1 & 10 & 25 & 0 \end{array} \]
\[ x^{2} + 10x + 25 = 0 \Rightarrow x = -5 \]
\[ x^{4} + 9x^{3} + 9x^{2} - 85x - 150 = (x + 5)^{2}(x + 2)(x - 3) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -216 | no |
| -1 | -64 | no |
| 2 | -196 | no |
| -2 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | -9 | -9 |
| Product of the roots | -150 | -150 |
Questions
What are the roots of x⁴ + 9x³ + 9x² - 85x - 150 = 0?
The solutions of x⁴ + 9x³ + 9x² - 85x - 150 = 0 are x = -5, -5, -2, 3 (repeated roots counted once each: -5, -2, 3).
How do you factor x⁴ + 9x³ + 9x² - 85x - 150?
(x + 5)(x + 2)(x − 3).
What is the sum of the roots?
-9 (it equals −9 by Vieta's formulas).