Solve x⁴ + 7x³ - 13x² - 175x - 300 = 0
The solutions of x⁴ + 7x³ - 13x² - 175x - 300 = 0 are x = -5, -4, -3, 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 -300, so the candidates are ±1, ±2, ±3, ±4, ±5, ±6, ±10, ±12, ±15, ±20, ±25, ±30, ±50, ±60, ±75, ±100, ±150, ±300.
- 2
Test the candidates until one gives 0. x = -3 is a root. Divide by (x + 3) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 3
x = -4 is a root. Divide by (x + 4) with synthetic division; the bottom row is the quotient and a 0 remainder.
- 4
What is left is the quadratic x² - 25 = 0. Solve it (factoring or the quadratic formula):
- 5
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{4} + 7x^{3} - 13x^{2} - 175x - 300 = (x + 5)(x + 4)(x + 3)(x - 5) = 0 \]
\[ x^{4} + 7x^{3} - 13x^{2} - 175x - 300 = 0 \]
\[ \begin{array}{c|ccccc} -3 & 1 & 7 & -13 & -175 & -300 \\ & 0 & -3 & -12 & 75 & 300 \\ \hline & 1 & 4 & -25 & -100 & 0 \end{array} \]
\[ \begin{array}{c|cccc} -4 & 1 & 4 & -25 & -100 \\ & 0 & -4 & 0 & 100 \\ \hline & 1 & 0 & -25 & 0 \end{array} \]
\[ x^{2} - 25 = 0 \Rightarrow x = 5,\ -5 \]
\[ x^{4} + 7x^{3} - 13x^{2} - 175x - 300 = (x + 5)(x + 4)(x + 3)(x - 5) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | -480 | no |
| -1 | -144 | no |
| 2 | -630 | no |
| -2 | -42 | no |
| 3 | -672 | no |
| -3 | 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 | -300 | -300 |
Questions
What are the roots of x⁴ + 7x³ - 13x² - 175x - 300 = 0?
The solutions of x⁴ + 7x³ - 13x² - 175x - 300 = 0 are x = -5, -4, -3, 5.
How do you factor x⁴ + 7x³ - 13x² - 175x - 300?
(x + 5)(x + 4)(x + 3)(x − 5).
What is the sum of the roots?
-7 (it equals −7 by Vieta's formulas).