Find the Roots/Zeros Using the Rational Roots Test x^3-5x^2+4x-20

$x^{3} - 5 x^{2} + 4 x - 20$

If a polynomial function has integer coefficients, then every rational zero will have the form $\frac{p}{q}$ where $p$ is a factor of the constant and $q$ is a factor of the leading coefficient.

$p = \pm 1, \pm 2, \pm 4, \pm 5, \pm 10, \pm 20$

$q = \pm 1$

Find every combination of $\pm \frac{p}{q}$ . These are the possible roots of the polynomial function.

$\pm 1, \pm 2, \pm 4, \pm 5, \pm 10, \pm 20$

Substitute the possible roots one by one into the polynomial to find the actual roots. Simplify to check if the value is $0$ , which means it is a root.

${(5)}^{3} - 5 {(5)}^{2} + 4 (5) - 20$

Simplify the expression. In this case, the expression is equal to $0$ so $x = 5$ is a root of the polynomial.

Simplify each term.

Raise $5$ to the power of $3$ .

$125 - 5 {(5)}^{2} + 4 (5) - 20$

Raise $5$ to the power of $2$ .

$125 - 5 \cdot 25 + 4 (5) - 20$

Multiply $- 5$ by $25$ .

$125 - 125 + 4 (5) - 20$

Multiply $4$ by $5$ .

$125 - 125 + 20 - 20$

Simplify by adding and subtracting.

Subtract $125$ from $125$ .

$0 + 20 - 20$

Add $0$ and $20$ .

$20 - 20$

Subtract $20$ from $20$ .

$0$

Since $5$ is a known root, divide the polynomial by $x - 5$ to find the quotient polynomial. This polynomial can then be used to find the remaining roots.

$\frac{x^{3} - 5 x^{2} + 4 x - 20}{x - 5}$

Next, find the roots of the remaining polynomial. The order of the polynomial has been reduced by $1$ .

Place the numbers representing the divisor and the dividend into a division-like configuration.

$5$	$1$	$- 5$	$4$	$- 20$
	\xa0			\xa0

The first number in the dividend $(1)$ is put into the first position of the result area (below the horizontal line).

$5$	$1$	$- 5$	$4$	$- 20$
	\xa0			\xa0
	$1$

Multiply the newest entry in the result $(1)$ by the divisor $(5)$ and place the result of $(5)$ under the next term in the dividend $(- 5)$ .

$5$	$1$	$- 5$	$4$	$- 20$
	\xa0	$5$		\xa0
	$1$

Add the product of the multiplication and the number from the dividend and put the result in the next position on the result line.

$5$	$1$	$- 5$	$4$	$- 20$
	\xa0	$5$		\xa0
	$1$	$0$

Multiply the newest entry in the result $(0)$ by the divisor $(5)$ and place the result of $(0)$ under the next term in the dividend $(4)$ .

$5$	$1$	$- 5$	$4$	$- 20$
	\xa0	$5$	$0$	\xa0
	$1$	$0$

Add the product of the multiplication and the number from the dividend and put the result in the next position on the result line.

$5$	$1$	$- 5$	$4$	$- 20$
	\xa0	$5$	$0$	\xa0
	$1$	$0$	$4$

Multiply the newest entry in the result $(4)$ by the divisor $(5)$ and place the result of $(20)$ under the next term in the dividend $(- 20)$ .

$5$	$1$	$- 5$	$4$	$- 20$
	\xa0	$5$	$0$	$20$
	$1$	$0$	$4$

Add the product of the multiplication and the number from the dividend and put the result in the next position on the result line.

$5$	$1$	$- 5$	$4$	$- 20$
	\xa0	$5$	$0$	$20$
	$1$	$0$	$4$	$0$

All numbers except the last become the coefficients of the quotient polynomial. The last value in the result line is the remainder.

$1 x^{2} + 0 x + 4$

Simplify the quotient polynomial.

$x^{2} + 4$

Solve the equation to find any remaining roots.

Subtract $4$ from both sides of the equation.

$x^{2} = - 4$

Take the square root of both sides of the equation to eliminate the exponent on the left side.

$x = \pm \sqrt{- 4}$

Simplify $\pm \sqrt{- 4}$ .

Rewrite $- 4$ as $- 1 (4)$ .

$x = \pm \sqrt{- 1 (4)}$

Rewrite $\sqrt{- 1 (4)}$ as $\sqrt{- 1} \cdot \sqrt{4}$ .

$x = \pm \sqrt{- 1} \cdot \sqrt{4}$

Rewrite $\sqrt{- 1}$ as $i$ .

$x = \pm i \cdot \sqrt{4}$

Rewrite $4$ as $2^{2}$ .

$x = \pm i \cdot \sqrt{2^{2}}$

Pull terms out from under the radical, assuming positive real numbers.

$x = \pm i \cdot 2$

Move $2$ to the left of $i$ .

$x = \pm 2 i$

The complete solution is the result of both the positive and negative portions of the solution.

First, use the positive value of the $\pm$ to find the first solution.

$x = 2 i$

Next, use the negative value of the $\pm$ to find the second solution.

$x = - 2 i$

The complete solution is the result of both the positive and negative portions of the solution.

$x = 2 i, - 2 i$

The polynomial can be written as a set of linear factors.

$(x - 5) (x - 2 i) (x + 2 i)$

These are the roots (zeros) of the polynomial $x^{3} - 5 x^{2} + 4 x - 20$ .

$x = 5, 2 i, - 2 i$

Find the Roots/Zeros Using the Rational Roots Test x^3-5x^2+4x-20

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