Precalculus Examples

$f (x) = \frac{x^{2} + 9 x - 9}{x - 9}$

Step 1

Find where the expression $\frac{x^{2} + 9 x - 9}{x - 9}$ is undefined.

$x = 9$

Step 2

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 3

Find $n$ and $m$ .

$n = 2$

$m = 1$

Step 4

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

No Horizontal Asymptotes

Step 5

Find the oblique asymptote using polynomial division.

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Step 5.1

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

$x$

$9$

$x^{2}$

$9 x$

$9$

Step 5.2

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

					$x$
	$x$	-	$9$		$x^{2}$	+	$9 x$	-	$9$

Step 5.3

Multiply the new quotient term by the divisor.

				$x$
$x$	-	$9$		$x^{2}$	+	$9 x$	-	$9$
			+	$x^{2}$	-	$9 x$

Step 5.4

The expression needs to be subtracted from the dividend, so change all the signs in $x^{2} - 9 x$

				$x$
$x$	-	$9$		$x^{2}$	+	$9 x$	-	$9$
			-	$x^{2}$	+	$9 x$

Step 5.5

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

				$x$
$x$	-	$9$		$x^{2}$	+	$9 x$	-	$9$
			-	$x^{2}$	+	$9 x$
					+	$18 x$

Step 5.6

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

				$x$
$x$	-	$9$		$x^{2}$	+	$9 x$	-	$9$
			-	$x^{2}$	+	$9 x$
					+	$18 x$	-	$9$

Step 5.7

Divide the highest order term in the dividend $18 x$ by the highest order term in divisor $x$ .

				$x$	+	$18$
$x$	-	$9$		$x^{2}$	+	$9 x$	-	$9$
			-	$x^{2}$	+	$9 x$
					+	$18 x$	-	$9$

Step 5.8

Multiply the new quotient term by the divisor.

				$x$	+	$18$
$x$	-	$9$		$x^{2}$	+	$9 x$	-	$9$
			-	$x^{2}$	+	$9 x$
					+	$18 x$	-	$9$
					+	$18 x$	-	$162$

Step 5.9

The expression needs to be subtracted from the dividend, so change all the signs in $18 x - 162$

				$x$	+	$18$
$x$	-	$9$		$x^{2}$	+	$9 x$	-	$9$
			-	$x^{2}$	+	$9 x$
					+	$18 x$	-	$9$
					-	$18 x$	+	$162$

Step 5.10

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

				$x$	+	$18$
$x$	-	$9$		$x^{2}$	+	$9 x$	-	$9$
			-	$x^{2}$	+	$9 x$
					+	$18 x$	-	$9$
					-	$18 x$	+	$162$
							+	$153$

Step 5.11

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

$x + 18 + \frac{153}{x - 9}$

Step 5.12

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

$y = x + 18$

Step 6

This is the set of all asymptotes.

Vertical Asymptotes: $x = 9$

No Horizontal Asymptotes

Oblique Asymptotes: $y = x + 18$

Step 7