Find the Roots/Zeros Using the Rational Roots Test f(x)=x^3+2x^2-5x-10

$f (x) = x^{3} + 2 x^{2} - 5 x - 10$

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.

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

$q = \pm 1$

Step 2

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

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

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.

${(- 2)}^{3} + 2 {(- 2)}^{2} - 5 \cdot - 2 - 10$

Step 4

Simplify the expression. In this case, the expression is equal to $0$ so $x = - 2$ is a root of the polynomial.

Simplify each term.

Raise $- 2$ to the power of $3$ .

$- 8 + 2 {(- 2)}^{2} - 5 \cdot - 2 - 10$

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

$- 8 + 2 \cdot 4 - 5 \cdot - 2 - 10$

Multiply $2$ by $4$ .

$- 8 + 8 - 5 \cdot - 2 - 10$

Multiply $- 5$ by $- 2$ .

$- 8 + 8 + 10 - 10$

Simplify by adding and subtracting.

Add $- 8$ and $8$ .

$0 + 10 - 10$

Add $0$ and $10$ .

$10 - 10$

Subtract $10$ from $10$ .

$0$

Step 5

Since $- 2$ is a known root, divide the polynomial by $x + 2$ to find the quotient polynomial. This polynomial can then be used to find the remaining roots.

$\frac{x^{3} + 2 x^{2} - 5 x - 10}{x + 2}$

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.

$- 2$	$1$	$2$	$- 5$	$- 10$
	\xa0			\xa0

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

$- 2$	$1$	$2$	$- 5$	$- 10$
	\xa0			\xa0
	$1$

Multiply the newest entry in the result $(1)$ by the divisor $(- 2)$ and place the result of $(- 2)$ under the next term in the dividend $(2)$ .

$- 2$	$1$	$2$	$- 5$	$- 10$
	\xa0	$- 2$		\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.

$- 2$	$1$	$2$	$- 5$	$- 10$
	\xa0	$- 2$		\xa0
	$1$	$0$

Multiply the newest entry in the result $(0)$ by the divisor $(- 2)$ and place the result of $(0)$ under the next term in the dividend $(- 5)$ .

$- 2$	$1$	$2$	$- 5$	$- 10$
	\xa0	$- 2$	$0$	\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.

$- 2$	$1$	$2$	$- 5$	$- 10$
	\xa0	$- 2$	$0$	\xa0
	$1$	$0$	$- 5$

Multiply the newest entry in the result $(- 5)$ by the divisor $(- 2)$ and place the result of $(10)$ under the next term in the dividend $(- 10)$ .

$- 2$	$1$	$2$	$- 5$	$- 10$
	\xa0	$- 2$	$0$	$10$
	$1$	$0$	$- 5$

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

$- 2$	$1$	$2$	$- 5$	$- 10$
	\xa0	$- 2$	$0$	$10$
	$1$	$0$	$- 5$	$0$

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

$1 x^{2} + 0 x - 5$

Simplify the quotient polynomial.

$x^{2} - 5$

Step 7

Solve the equation to find any remaining roots.

Add $5$ to both sides of the equation.

$x^{2} = 5$

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

$x = \pm \sqrt{5}$

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.

$x = \sqrt{5}$

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

$x = - \sqrt{5}$

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

$x = \sqrt{5}, - \sqrt{5}$

Step 8

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

$(x + 2) (x - \sqrt{5}) (x + \sqrt{5})$

Step 9

These are the roots (zeros) of the polynomial $x^{3} + 2 x^{2} - 5 x - 10$ .

$x = - 2, \sqrt{5}, - \sqrt{5}$

Step 10

The result can be shown in multiple forms.

Exact Form:

$x = - 2, \sqrt{5}, - \sqrt{5}$

Decimal Form:

$x = - 2, 2.23606797 \dots, - 2.23606797 \dots$

Step 11

Find the Roots/Zeros Using the Rational Roots Test f(x)=x^3+2x^2-5x-10

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