Solve x⁴ - 18x³ + 120x² - 350x + 375 = 0
The solutions of x⁴ - 18x³ + 120x² - 350x + 375 = 0 are x = 3, 5, 5, 5 (repeated roots counted once each: 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 375, so the candidates are ±1, ±3, ±5, ±15, ±25, ±75, ±125, ±375.
- 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 = 5 is a root. Divide by (x − 5) 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
\[ x^{4} - 18x^{3} + 120x^{2} - 350x + 375 = (x - 3)(x - 5)^{3} = 0 \]
\[ x^{4} - 18x^{3} + 120x^{2} - 350x + 375 = 0 \]
\[ \begin{array}{c|ccccc} 3 & 1 & -18 & 120 & -350 & 375 \\ & 0 & 3 & -45 & 225 & -375 \\ \hline & 1 & -15 & 75 & -125 & 0 \end{array} \]
\[ \begin{array}{c|cccc} 5 & 1 & -15 & 75 & -125 \\ & 0 & 5 & -50 & 125 \\ \hline & 1 & -10 & 25 & 0 \end{array} \]
\[ x^{2} - 10x + 25 = 0 \Rightarrow x = 5 \]
\[ x^{4} - 18x^{3} + 120x^{2} - 350x + 375 = (x - 3)(x - 5)^{3} \]Candidates tested (computed)
| Candidate x | p(x) | Root? |
|---|---|---|
| 1 | 128 | no |
| -1 | 864 | no |
| 3 | 0 | root |
Vieta's formulas check the answer (computed)
| Relation | From the roots | From the coefficients |
|---|---|---|
| Sum of the roots | 18 | 18 |
| Product of the roots | 375 | 375 |
Questions
What are the roots of x⁴ - 18x³ + 120x² - 350x + 375 = 0?
The solutions of x⁴ - 18x³ + 120x² - 350x + 375 = 0 are x = 3, 5, 5, 5 (repeated roots counted once each: 3, 5).
How do you factor x⁴ - 18x³ + 120x² - 350x + 375?
(x − 3)(x − 5).
What is the sum of the roots?
18 (it equals −(-18) by Vieta's formulas).