Solve x⁴ - 4x³ - 21x² + 64x + 80 = 0
The solutions of x⁴ - 4x³ - 21x² + 64x + 80 = 0 are x = -4, -1, 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 80, so the candidates are ±1, ±2, ±4, ±5, ±8, ±10, ±16, ±20, ±40, ±80.
- 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
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 - x - 20 = 0. Solve it (factoring or the quadratic formula):
- 5
So the polynomial factors as
LaTeX source of the formulas on this page
\[ x^{4} - 4x^{3} - 21x^{2} + 64x + 80 = (x + 4)(x + 1)(x - 4)(x - 5) = 0 \]
\[ x^{4} - 4x^{3} - 21x^{2} + 64x + 80 = 0 \]
\[ \begin{array}{c|ccccc} -1 & 1 & -4 & -21 & 64 & 80 \\ & 0 & -1 & 5 & 16 & -80 \\ \hline & 1 & -5 & -16 & 80 & 0 \end{array} \]
\[ \begin{array}{c|cccc} 4 & 1 & -5 & -16 & 80 \\ & 0 & 4 & -4 & -80 \\ \hline & 1 & -1 & -20 & 0 \end{array} \]
\[ x^{2} - x - 20 = 0 \Rightarrow x = 5,\ -4 \]
\[ x^{4} - 4x^{3} - 21x^{2} + 64x + 80 = (x + 4)(x + 1)(x - 4)(x - 5) \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | 120 | no |
| -1 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | 4 | 4 |
| Product of the roots | 80 | 80 |
Questions
What are the roots of x⁴ - 4x³ - 21x² + 64x + 80 = 0?
The solutions of x⁴ - 4x³ - 21x² + 64x + 80 = 0 are x = -4, -1, 4, 5.
How do you factor x⁴ - 4x³ - 21x² + 64x + 80?
(x + 4)(x + 1)(x − 4)(x − 5).
What is the sum of the roots?
4 (it equals −(-4) by Vieta's formulas).