Precalculus Examples

$x^{3} - 5 x^{2} + 2 x + 8 = 0$

Step 1

Factor the left side of the equation.

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Step 1.1

Factor $x^{3} - 5 x^{2} + 2 x + 8$ using the rational roots test.

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Step 1.1.1

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.

$p = \pm 1, \pm 8, \pm 2, \pm 4$

$q = \pm 1$

Step 1.1.2

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

$\pm 1, \pm 8, \pm 2, \pm 4$

Step 1.1.3

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

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Step 1.1.3.1

Substitute $- 1$ into the polynomial.

${(- 1)}^{3} - 5 {(- 1)}^{2} + 2 \cdot - 1 + 8$

Step 1.1.3.2

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

$- 1 - 5 {(- 1)}^{2} + 2 \cdot - 1 + 8$

Step 1.1.3.3

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

$- 1 - 5 \cdot 1 + 2 \cdot - 1 + 8$

Step 1.1.3.4

Multiply $- 5$ by $1$ .

$- 1 - 5 + 2 \cdot - 1 + 8$

Step 1.1.3.5

Subtract $5$ from $- 1$ .

$- 6 + 2 \cdot - 1 + 8$

Step 1.1.3.6

Multiply $2$ by $- 1$ .

$- 6 - 2 + 8$

Step 1.1.3.7

Subtract $2$ from $- 6$ .

$- 8 + 8$

Step 1.1.3.8

Add $- 8$ and $8$ .

$0$

Step 1.1.4

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.

$\frac{x^{3} - 5 x^{2} + 2 x + 8}{x + 1}$

Step 1.1.5

Divide $x^{3} - 5 x^{2} + 2 x + 8$ by $x + 1$ .

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Step 1.1.5.1

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

$x$

$1$

$x^{3}$

$5 x^{2}$

$2 x$

$8$

Step 1.1.5.2

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

					$x^{2}$
	$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$

Step 1.1.5.3

Multiply the new quotient term by the divisor.

				$x^{2}$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			+	$x^{3}$	+	$x^{2}$

Step 1.1.5.4

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

				$x^{2}$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$

Step 1.1.5.5

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

				$x^{2}$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$

Step 1.1.5.6

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

				$x^{2}$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$

Step 1.1.5.7

Divide the highest order term in the dividend $- 6 x^{2}$ by the highest order term in divisor $x$ .

				$x^{2}$	-	$6 x$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$

Step 1.1.5.8

Multiply the new quotient term by the divisor.

				$x^{2}$	-	$6 x$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$
					-	$6 x^{2}$	-	$6 x$

Step 1.1.5.9

The expression needs to be subtracted from the dividend, so change all the signs in $- 6 x^{2} - 6 x$

				$x^{2}$	-	$6 x$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$
					+	$6 x^{2}$	+	$6 x$

Step 1.1.5.10

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

				$x^{2}$	-	$6 x$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$
					+	$6 x^{2}$	+	$6 x$
							+	$8 x$

Step 1.1.5.11

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

				$x^{2}$	-	$6 x$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$
					+	$6 x^{2}$	+	$6 x$
							+	$8 x$	+	$8$

Step 1.1.5.12

Divide the highest order term in the dividend $8 x$ by the highest order term in divisor $x$ .

				$x^{2}$	-	$6 x$	+	$8$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$
					+	$6 x^{2}$	+	$6 x$
							+	$8 x$	+	$8$

Step 1.1.5.13

Multiply the new quotient term by the divisor.

				$x^{2}$	-	$6 x$	+	$8$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$
					+	$6 x^{2}$	+	$6 x$
							+	$8 x$	+	$8$
							+	$8 x$	+	$8$

Step 1.1.5.14

The expression needs to be subtracted from the dividend, so change all the signs in $8 x + 8$

				$x^{2}$	-	$6 x$	+	$8$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$
					+	$6 x^{2}$	+	$6 x$
							+	$8 x$	+	$8$
							-	$8 x$	-	$8$

Step 1.1.5.15

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

				$x^{2}$	-	$6 x$	+	$8$
$x$	+	$1$		$x^{3}$	-	$5 x^{2}$	+	$2 x$	+	$8$
			-	$x^{3}$	-	$x^{2}$
					-	$6 x^{2}$	+	$2 x$
					+	$6 x^{2}$	+	$6 x$
							+	$8 x$	+	$8$
							-	$8 x$	-	$8$
										$0$

Step 1.1.5.16

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

$x^{2} - 6 x + 8$

Step 1.1.6

Write $x^{3} - 5 x^{2} + 2 x + 8$ as a set of factors.

$(x + 1) (x^{2} - 6 x + 8) = 0$

Step 1.2

Factor $x^{2} - 6 x + 8$ using the AC method.

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Step 1.2.1

Factor $x^{2} - 6 x + 8$ using the AC method.

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Step 1.2.1.1

Consider the form $x^{2} + b x + c$ . Find a pair of integers whose product is $c$ and whose sum is $b$ . In this case, whose product is $8$ and whose sum is $- 6$ .

$- 4, - 2$

Step 1.2.1.2

Write the factored form using these integers.

$(x + 1) ((x - 4) (x - 2)) = 0$

Step 1.2.2

Remove unnecessary parentheses.

$(x + 1) (x - 4) (x - 2) = 0$

Step 2

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

$x + 1 = 0$

$x - 4 = 0$

$x - 2 = 0$

Step 3

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

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Step 3.1

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

$x + 1 = 0$

Step 3.2

Subtract $1$ from both sides of the equation.

$x = - 1$

Step 4

Set $x - 4$ equal to $0$ and solve for $x$ .

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Step 4.1

Set $x - 4$ equal to $0$ .

$x - 4 = 0$

Step 4.2

Add $4$ to both sides of the equation.

$x = 4$

Step 5

Set $x - 2$ equal to $0$ and solve for $x$ .

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Step 5.1

Set $x - 2$ equal to $0$ .

$x - 2 = 0$

Step 5.2

Add $2$ to both sides of the equation.

$x = 2$

Step 6

The final solution is all the values that make $(x + 1) (x - 4) (x - 2) = 0$ true.

$x = - 1, 4, 2$