Find the Roots/Zeros Using the Rational Roots Test x^3+9x^2-13x-21

$x^{3} + 9 x^{2} - 13 x - 21$

Step 1

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.

$p = \pm 1, \pm 3, \pm 7, \pm 21$

$q = \pm 1$

Step 2

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

$\pm 1, \pm 3, \pm 7, \pm 21$

Step 3

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.

${(- 1)}^{3} + 9 {(- 1)}^{2} - 13 \cdot - 1 - 21$

Step 4

Simplify the expression. In this case, the expression is equal to $0$ so $x = - 1$ is a root of the polynomial.

Simplify each term.

Raise $- 1$ to the power of $3$ .

$- 1 + 9 {(- 1)}^{2} - 13 \cdot - 1 - 21$

Raise $- 1$ to the power of $2$ .

$- 1 + 9 \cdot 1 - 13 \cdot - 1 - 21$

Multiply $9$ by $1$ .

$- 1 + 9 - 13 \cdot - 1 - 21$

Multiply $- 13$ by $- 1$ .

$- 1 + 9 + 13 - 21$

Simplify by adding and subtracting.

Add $- 1$ and $9$ .

$8 + 13 - 21$

Add $8$ and $13$ .

$21 - 21$

Subtract $21$ from $21$ .

$0$

Step 5

Since $- 1$ is a known root, divide the polynomial by $x + 1$ to find the quotient polynomial. This polynomial can then be used to find the remaining roots.

$\frac{x^{3} + 9 x^{2} - 13 x - 21}{x + 1}$

Step 6

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.

$- 1$	$1$	$9$	$- 13$	$- 21$
	\xa0			\xa0

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

$- 1$	$1$	$9$	$- 13$	$- 21$
	\xa0			\xa0
	$1$

Multiply the newest entry in the result $(1)$ by the divisor $(- 1)$ and place the result of $(- 1)$ under the next term in the dividend $(9)$ .

$- 1$	$1$	$9$	$- 13$	$- 21$
	\xa0	$- 1$		\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.

$- 1$	$1$	$9$	$- 13$	$- 21$
	\xa0	$- 1$		\xa0
	$1$	$8$

Multiply the newest entry in the result $(8)$ by the divisor $(- 1)$ and place the result of $(- 8)$ under the next term in the dividend $(- 13)$ .

$- 1$	$1$	$9$	$- 13$	$- 21$
	\xa0	$- 1$	$- 8$	\xa0
	$1$	$8$

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

$- 1$	$1$	$9$	$- 13$	$- 21$
	\xa0	$- 1$	$- 8$	\xa0
	$1$	$8$	$- 21$

Multiply the newest entry in the result $(- 21)$ by the divisor $(- 1)$ and place the result of $(21)$ under the next term in the dividend $(- 21)$ .

$- 1$	$1$	$9$	$- 13$	$- 21$
	\xa0	$- 1$	$- 8$	$21$
	$1$	$8$	$- 21$

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

$- 1$	$1$	$9$	$- 13$	$- 21$
	\xa0	$- 1$	$- 8$	$21$
	$1$	$8$	$- 21$	$0$

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

$1 x^{2} + 8 x - 21$

Simplify the quotient polynomial.

$x^{2} + 8 x - 21$

Step 7

Solve the equation to find any remaining roots.

Use the quadratic formula to find the solutions.

$\frac{- b \pm \sqrt{b^{2} - 4 (a c)}}{2 a}$

Substitute the values $a = 1$ , $b = 8$ , and $c = - 21$ into the quadratic formula and solve for $x$ .

$\frac{- 8 \pm \sqrt{8^{2} - 4 \cdot (1 \cdot - 21)}}{2 \cdot 1}$

Simplify.

Simplify the numerator.

Raise $8$ to the power of $2$ .

$x = \frac{- 8 \pm \sqrt{64 - 4 \cdot 1 \cdot - 21}}{2 \cdot 1}$

Multiply $- 4 \cdot 1 \cdot - 21$ .

Multiply $- 4$ by $1$ .

$x = \frac{- 8 \pm \sqrt{64 - 4 \cdot - 21}}{2 \cdot 1}$

Multiply $- 4$ by $- 21$ .

$x = \frac{- 8 \pm \sqrt{64 + 84}}{2 \cdot 1}$

Add $64$ and $84$ .

$x = \frac{- 8 \pm \sqrt{148}}{2 \cdot 1}$

Rewrite $148$ as $2^{2} \cdot 37$ .

Factor $4$ out of $148$ .

$x = \frac{- 8 \pm \sqrt{4 (37)}}{2 \cdot 1}$

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

$x = \frac{- 8 \pm \sqrt{2^{2} \cdot 37}}{2 \cdot 1}$

Pull terms out from under the radical.

$x = \frac{- 8 \pm 2 \sqrt{37}}{2 \cdot 1}$

Multiply $2$ by $1$ .

$x = \frac{- 8 \pm 2 \sqrt{37}}{2}$

Simplify $\frac{- 8 \pm 2 \sqrt{37}}{2}$ .

$x = - 4 \pm \sqrt{37}$

The final answer is the combination of both solutions.

$x = - 4 + \sqrt{37}, - 4 - \sqrt{37}$

Step 8

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

$(x + 1) (x + 4 - \sqrt{37}) (x + 4 + \sqrt{37})$

Step 9

These are the roots (zeros) of the polynomial $x^{3} + 9 x^{2} - 13 x - 21$ .

$x = - 1, - 4 + \sqrt{37}, - 4 - \sqrt{37}$

Step 10

The result can be shown in multiple forms.

Exact Form:

$x = - 1, - 4 + \sqrt{37}, - 4 - \sqrt{37}$

Decimal Form:

$x = - 1, 2.08276253 \dots, - 10.08276253 \dots$

Step 11

Find the Roots/Zeros Using the Rational Roots Test x^3+9x^2-13x-21

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