Precalculus Examples

$f (x) = \frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x^{2} + 14 x + 45}$

Step 1

Find where the expression $\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x^{2} + 14 x + 45}$ is undefined.

$x = - 9, x = - 5$

Step 2

Since $\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x^{2} + 14 x + 45}$ $\to$ $- \infty$ as $x$ $\to$ $- 9$ from the left and $\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x^{2} + 14 x + 45}$ $\to$ $\infty$ as $x$ $\to$ $- 9$ from the right, then $x = - 9$ is a vertical asymptote.

$x = - 9$

Step 3

Consider the rational function $R (x) = \frac{a x^{n}}{b x^{m}}$ where $n$ is the degree of the numerator and $m$ is the degree of the denominator.

1. If $n < m$ , then the x-axis, $y = 0$ , is the horizontal asymptote.

2. If $n = m$ , then the horizontal asymptote is the line $y = \frac{a}{b}$ .

3. If $n > m$ , then there is no horizontal asymptote (there is an oblique asymptote).

Step 4

Find $n$ and $m$ .

$n = 3$

$m = 2$

Step 5

Since $n > m$ , there is no horizontal asymptote.

No Horizontal Asymptotes

Step 6

Find the oblique asymptote using polynomial division.

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Step 6.1

Factor $x^{2} + 14 x + 45$ using the AC method.

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Step 6.1.1

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 $45$ and whose sum is $14$ .

$5, 9$

Step 6.1.2

Write the factored form using these integers.

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{(x + 5) (x + 9)}$

Step 6.2

Expand $(x + 5) (x + 9)$ .

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Step 6.2.1

Apply the distributive property.

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x (x + 9) + 5 (x + 9)}$

Step 6.2.2

Apply the distributive property.

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x \cdot x + x \cdot 9 + 5 (x + 9)}$

Step 6.2.3

Apply the distributive property.

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x \cdot x + x \cdot 9 + 5 x + 5 \cdot 9}$

Step 6.2.4

Reorder $x$ and $9$ .

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x \cdot x + 9 x + 5 x + 5 \cdot 9}$

Step 6.2.5

Raise $x$ to the power of $1$ .

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x \cdot x + 9 x + 5 x + 5 \cdot 9}$

Step 6.2.6

Raise $x$ to the power of $1$ .

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x \cdot x + 9 x + 5 x + 5 \cdot 9}$

Step 6.2.7

Use the power rule $a^{m} a^{n} = a^{m + n}$ to combine exponents.

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x^{1 + 1} + 9 x + 5 x + 5 \cdot 9}$

Step 6.2.8

Add $1$ and $1$ .

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x^{2} + 9 x + 5 x + 5 \cdot 9}$

Step 6.2.9

Multiply $5$ by $9$ .

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x^{2} + 9 x + 5 x + 45}$

Step 6.2.10

Add $9 x$ and $5 x$ .

$\frac{2 x^{3} + 24 x^{2} + 35 x - 175}{x^{2} + 14 x + 45}$

Step 6.3

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

$x^{2}$

$14 x$

$45$

$2 x^{3}$

$24 x^{2}$

$35 x$

$175$

Step 6.4

Divide the highest order term in the dividend $2 x^{3}$ by the highest order term in divisor $x^{2}$ .

$2 x$

$x^{2}$

$14 x$

$45$

$2 x^{3}$

$24 x^{2}$

$35 x$

$175$

Step 6.5

Multiply the new quotient term by the divisor.

$2 x$

$x^{2}$

$14 x$

$45$

$2 x^{3}$

$24 x^{2}$

$35 x$

$175$

$2 x^{3}$

$28 x^{2}$

$90 x$

Step 6.6

The expression needs to be subtracted from the dividend, so change all the signs in $2 x^{3} + 28 x^{2} + 90 x$

$2 x$

$x^{2}$

$14 x$

$45$

$2 x^{3}$

$24 x^{2}$

$35 x$

$175$

$2 x^{3}$

$28 x^{2}$

$90 x$

Step 6.7

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

$2 x$

$x^{2}$

$14 x$

$45$

$2 x^{3}$

$24 x^{2}$

$35 x$

$175$

$2 x^{3}$

$28 x^{2}$

$90 x$

$4 x^{2}$

$55 x$

Step 6.8

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

						$2 x$
$x^{2}$	+	$14 x$	+	$45$		$2 x^{3}$	+	$24 x^{2}$	+	$35 x$	-	$175$
					-	$2 x^{3}$	-	$28 x^{2}$	-	$90 x$
							-	$4 x^{2}$	-	$55 x$	-	$175$

Step 6.9

Divide the highest order term in the dividend $- 4 x^{2}$ by the highest order term in divisor $x^{2}$ .

						$2 x$	-	$4$
$x^{2}$	+	$14 x$	+	$45$		$2 x^{3}$	+	$24 x^{2}$	+	$35 x$	-	$175$
					-	$2 x^{3}$	-	$28 x^{2}$	-	$90 x$
							-	$4 x^{2}$	-	$55 x$	-	$175$

Step 6.10

Multiply the new quotient term by the divisor.

						$2 x$	-	$4$
$x^{2}$	+	$14 x$	+	$45$		$2 x^{3}$	+	$24 x^{2}$	+	$35 x$	-	$175$
					-	$2 x^{3}$	-	$28 x^{2}$	-	$90 x$
							-	$4 x^{2}$	-	$55 x$	-	$175$
							-	$4 x^{2}$	-	$56 x$	-	$180$

Step 6.11

The expression needs to be subtracted from the dividend, so change all the signs in $- 4 x^{2} - 56 x - 180$

$2 x$

$4$

$x^{2}$

$14 x$

$45$

$2 x^{3}$

$24 x^{2}$

$35 x$

$175$

$2 x^{3}$

$28 x^{2}$

$90 x$

$4 x^{2}$

$55 x$

$175$

$4 x^{2}$

$56 x$

$180$

Step 6.12

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

$2 x$

$4$

$x^{2}$

$14 x$

$45$

$2 x^{3}$

$24 x^{2}$

$35 x$

$175$

$2 x^{3}$

$28 x^{2}$

$90 x$

$4 x^{2}$

$55 x$

$175$

$4 x^{2}$

$56 x$

$180$

$x$

$5$

Step 6.13

The final answer is the quotient plus the remainder over the divisor.

$2 x - 4 + \frac{x + 5}{x^{2} + 14 x + 45}$

Step 6.14

The oblique asymptote is the polynomial portion of the long division result.

$y = 2 x - 4$

Step 7

This is the set of all asymptotes.

Vertical Asymptotes: $x = - 9$

No Horizontal Asymptotes

Oblique Asymptotes: $y = 2 x - 4$

Step 8