Find the Roots/Zeros Using the Rational Roots Test x^4-8x^3+7x^2+42x+26

$x^{4} - 8 x^{3} + 7 x^{2} + 42 x + 26$

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.

$p = \pm 1, \pm 2, \pm 13, \pm 26$

$q = \pm 1$

Step 2

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

$\pm 1, \pm 2, \pm 13, \pm 26$

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.

${(- 1)}^{4} - 8 {(- 1)}^{3} + 7 {(- 1)}^{2} + 42 (- 1) + 26$

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 $4$ .

$1 - 8 {(- 1)}^{3} + 7 {(- 1)}^{2} + 42 (- 1) + 26$

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

$1 - 8 \cdot - 1 + 7 {(- 1)}^{2} + 42 (- 1) + 26$

Multiply $- 8$ by $- 1$ .

$1 + 8 + 7 {(- 1)}^{2} + 42 (- 1) + 26$

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

$1 + 8 + 7 \cdot 1 + 42 (- 1) + 26$

Multiply $7$ by $1$ .

$1 + 8 + 7 + 42 (- 1) + 26$

Multiply $42$ by $- 1$ .

$1 + 8 + 7 - 42 + 26$

Simplify by adding and subtracting.

Add $1$ and $8$ .

$9 + 7 - 42 + 26$

Add $9$ and $7$ .

$16 - 42 + 26$

Subtract $42$ from $16$ .

$- 26 + 26$

Add $- 26$ and $26$ .

$0$

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.

$\frac{x^{4} - 8 x^{3} + 7 x^{2} + 42 x + 26}{x + 1}$

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.

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\xa0				\xa0

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

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\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 $(- 8)$ .

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\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.

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\xa0	$- 1$			\xa0
	$1$	$- 9$

Multiply the newest entry in the result $(- 9)$ by the divisor $(- 1)$ and place the result of $(9)$ under the next term in the dividend $(7)$ .

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\xa0	$- 1$	$9$		\xa0
	$1$	$- 9$

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

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\xa0	$- 1$	$9$		\xa0
	$1$	$- 9$	$16$

Multiply the newest entry in the result $(16)$ by the divisor $(- 1)$ and place the result of $(- 16)$ under the next term in the dividend $(42)$ .

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\xa0	$- 1$	$9$	$- 16$	\xa0
	$1$	$- 9$	$16$

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

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\xa0	$- 1$	$9$	$- 16$	\xa0
	$1$	$- 9$	$16$	$26$

Multiply the newest entry in the result $(26)$ by the divisor $(- 1)$ and place the result of $(- 26)$ under the next term in the dividend $(26)$ .

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\xa0	$- 1$	$9$	$- 16$	$- 26$
	$1$	$- 9$	$16$	$26$

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

$- 1$	$1$	$- 8$	$7$	$42$	$26$
	\xa0	$- 1$	$9$	$- 16$	$- 26$
	$1$	$- 9$	$16$	$26$	$0$

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

$1 x^{3} + - 9 x^{2} + (16) x + 26$

Simplify the quotient polynomial.

$x^{3} - 9 x^{2} + 16 x + 26$

Step 7

Solve the equation to find any remaining roots.

Factor $x^{3} - 9 x^{2} + 16 x + 26$ using the rational roots test.

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.

$p = \pm 1, \pm 26, \pm 2, \pm 13 q = \pm 1$

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

$\pm 1, \pm 26, \pm 2, \pm 13$

Substitute $- 1$ and simplify the expression. In this case, the expression is equal to $0$ so $- 1$ is a root of the polynomial.

Substitute $- 1$ into the polynomial.

${(- 1)}^{3} - 9 {(- 1)}^{2} + 16 \cdot - 1 + 26$

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

$- 1 - 9 {(- 1)}^{2} + 16 \cdot - 1 + 26$

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

$- 1 - 9 \cdot 1 + 16 \cdot - 1 + 26$

Multiply $- 9$ by $1$ .

$- 1 - 9 + 16 \cdot - 1 + 26$

Subtract $9$ from $- 1$ .

$- 10 + 16 \cdot - 1 + 26$

Multiply $16$ by $- 1$ .

$- 10 - 16 + 26$

Subtract $16$ from $- 10$ .

$- 26 + 26$

Add $- 26$ and $26$ .

$0$

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.

$\frac{x^{3} - 9 x^{2} + 16 x + 26}{x + 1}$

Divide $x^{3} - 9 x^{2} + 16 x + 26$ by $x + 1$ .

Set up the polynomials to be divided. If there is not a term for every exponent, insert one with a value of $0$ .

$x$

$1$

$x^{3}$

$9 x^{2}$

$16 x$

$26$

Divide the highest order term in the dividend $x^{3}$ by the highest order term in divisor $x$ .

					$x^{2}$
	$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$

Multiply the new quotient term by the divisor.

				$x^{2}$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			+	$x^{3}$	+	$x^{2}$

The expression needs to be subtracted from the dividend, so change all the signs in $x^{3} + x^{2}$

				$x^{2}$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$

After changing the signs, add the last dividend from the multiplied polynomial to find the new dividend.

				$x^{2}$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$

Pull the next terms from the original dividend down into the current dividend.

				$x^{2}$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$

Divide the highest order term in the dividend $- 10 x^{2}$ by the highest order term in divisor $x$ .

				$x^{2}$	-	$10 x$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$

Multiply the new quotient term by the divisor.

				$x^{2}$	-	$10 x$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$
					-	$10 x^{2}$	-	$10 x$

The expression needs to be subtracted from the dividend, so change all the signs in $- 10 x^{2} - 10 x$

				$x^{2}$	-	$10 x$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$
					+	$10 x^{2}$	+	$10 x$

After changing the signs, add the last dividend from the multiplied polynomial to find the new dividend.

				$x^{2}$	-	$10 x$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$
					+	$10 x^{2}$	+	$10 x$
							+	$26 x$

Pull the next terms from the original dividend down into the current dividend.

				$x^{2}$	-	$10 x$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$
					+	$10 x^{2}$	+	$10 x$
							+	$26 x$	+	$26$

Divide the highest order term in the dividend $26 x$ by the highest order term in divisor $x$ .

				$x^{2}$	-	$10 x$	+	$26$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$
					+	$10 x^{2}$	+	$10 x$
							+	$26 x$	+	$26$

Multiply the new quotient term by the divisor.

				$x^{2}$	-	$10 x$	+	$26$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$
					+	$10 x^{2}$	+	$10 x$
							+	$26 x$	+	$26$
							+	$26 x$	+	$26$

The expression needs to be subtracted from the dividend, so change all the signs in $26 x + 26$

				$x^{2}$	-	$10 x$	+	$26$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$
					+	$10 x^{2}$	+	$10 x$
							+	$26 x$	+	$26$
							-	$26 x$	-	$26$

After changing the signs, add the last dividend from the multiplied polynomial to find the new dividend.

				$x^{2}$	-	$10 x$	+	$26$
$x$	+	$1$		$x^{3}$	-	$9 x^{2}$	+	$16 x$	+	$26$
			-	$x^{3}$	-	$x^{2}$
					-	$10 x^{2}$	+	$16 x$
					+	$10 x^{2}$	+	$10 x$
							+	$26 x$	+	$26$
							-	$26 x$	-	$26$
										$0$

Since the remander is $0$ , the final answer is the quotient.

$x^{2} - 10 x + 26$

Write $x^{3} - 9 x^{2} + 16 x + 26$ as a set of factors.

$(x + 1) (x^{2} - 10 x + 26) = 0$

If any individual factor on the left side of the equation is equal to $0$ , the entire expression will be equal to $0$ .

$x + 1 = 0$

$x^{2} - 10 x + 26 = 0$

Set $x + 1$ equal to $0$ and solve for $x$ .

Set $x + 1$ equal to $0$ .

$x + 1 = 0$

Subtract $1$ from both sides of the equation.

$x = - 1$

Set $x^{2} - 10 x + 26$ equal to $0$ and solve for $x$ .

Set $x^{2} - 10 x + 26$ equal to $0$ .

$x^{2} - 10 x + 26 = 0$

Solve $x^{2} - 10 x + 26 = 0$ for $x$ .

Use the quadratic formula to find the solutions.

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

Substitute the values $a = 1$ , $b = - 10$ , and $c = 26$ into the quadratic formula and solve for $x$ .

$\frac{10 \pm \sqrt{{(- 10)}^{2} - 4 \cdot (1 \cdot 26)}}{2 \cdot 1}$

Simplify.

Simplify the numerator.

Raise $- 10$ to the power of $2$ .

$x = \frac{10 \pm \sqrt{100 - 4 \cdot 1 \cdot 26}}{2 \cdot 1}$

Multiply $- 4 \cdot 1 \cdot 26$ .

Multiply $- 4$ by $1$ .

$x = \frac{10 \pm \sqrt{100 - 4 \cdot 26}}{2 \cdot 1}$

Multiply $- 4$ by $26$ .

$x = \frac{10 \pm \sqrt{100 - 104}}{2 \cdot 1}$

Subtract $104$ from $100$ .

$x = \frac{10 \pm \sqrt{- 4}}{2 \cdot 1}$

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

$x = \frac{10 \pm \sqrt{- 1 \cdot 4}}{2 \cdot 1}$

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

$x = \frac{10 \pm \sqrt{- 1} \cdot \sqrt{4}}{2 \cdot 1}$

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

$x = \frac{10 \pm i \cdot \sqrt{4}}{2 \cdot 1}$

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

$x = \frac{10 \pm i \cdot \sqrt{2^{2}}}{2 \cdot 1}$

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

$x = \frac{10 \pm i \cdot 2}{2 \cdot 1}$

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

$x = \frac{10 \pm 2 i}{2 \cdot 1}$

Multiply $2$ by $1$ .

$x = \frac{10 \pm 2 i}{2}$

Simplify $\frac{10 \pm 2 i}{2}$ .

$x = 5 \pm i$

The final answer is the combination of both solutions.

$x = 5 + i, 5 - i$

The final solution is all the values that make $(x + 1) (x^{2} - 10 x + 26) = 0$ true.

$x = - 1, 5 + i, 5 - i$

Step 8

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

$(x + 1) (x - 5 - i) (x - 5 + i)$

Step 9

These are the roots (zeros) of the polynomial $x^{4} - 8 x^{3} + 7 x^{2} + 42 x + 26$ .

$x = - 1, 5 + i, 5 - i$

Step 10

Find the Roots/Zeros Using the Rational Roots Test x^4-8x^3+7x^2+42x+26

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