Precalculus Examples

$f (x) = \frac{x^{3}}{2 x^{2} - 8}$

Step 1

Find where the expression $\frac{x^{3}}{2 x^{2} - 8}$ is undefined.

$x = - 2, x = 2$

Step 2

Since $\frac{x^{3}}{2 x^{2} - 8}$ $\to$ $- \infty$ as $x$ $\to$ $- 2$ from the left and $\frac{x^{3}}{2 x^{2} - 8}$ $\to$ $\infty$ as $x$ $\to$ $- 2$ from the right, then $x = - 2$ is a vertical asymptote.

$x = - 2$

Step 3

Since $\frac{x^{3}}{2 x^{2} - 8}$ $\to$ $- \infty$ as $x$ $\to$ $2$ from the left and $\frac{x^{3}}{2 x^{2} - 8}$ $\to$ $\infty$ as $x$ $\to$ $2$ from the right, then $x = 2$ is a vertical asymptote.

$x = 2$

Step 4

List all of the vertical asymptotes:

$x = - 2, 2$

Step 5

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 6

Find $n$ and $m$ .

$n = 3$

$m = 2$

Step 7

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

No Horizontal Asymptotes

Step 8

Find the oblique asymptote using polynomial division.

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Step 8.1

Simplify the denominator.

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Step 8.1.1

Factor $2$ out of $2 x^{2} - 8$ .

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Step 8.1.1.1

Factor $2$ out of $2 x^{2}$ .

$\frac{x^{3}}{2 (x^{2}) - 8}$

Step 8.1.1.2

Factor $2$ out of $- 8$ .

$\frac{x^{3}}{2 x^{2} + 2 \cdot - 4}$

Step 8.1.1.3

Factor $2$ out of $2 x^{2} + 2 \cdot - 4$ .

$\frac{x^{3}}{2 (x^{2} - 4)}$

Step 8.1.2

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

$\frac{x^{3}}{2 (x^{2} - 2^{2})}$

Step 8.1.3

Since both terms are perfect squares, factor using the difference of squares formula, $a^{2} - b^{2} = (a + b) (a - b)$ where $a = x$ and $b = 2$ .

$\frac{x^{3}}{2 (x + 2) (x - 2)}$

Step 8.2

Expand $2 (x + 2) (x - 2)$ .

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Step 8.2.1

Apply the distributive property.

$\frac{x^{3}}{(2 x + 2 \cdot 2) (x - 2)}$

Step 8.2.2

Apply the distributive property.

$\frac{x^{3}}{2 x (x - 2) + 2 \cdot (2 (x - 2))}$

Step 8.2.3

Apply the distributive property.

$\frac{x^{3}}{2 x \cdot x + 2 x \cdot - 2 + 2 \cdot (2 (x - 2))}$

Step 8.2.4

Apply the distributive property.

$\frac{x^{3}}{2 x \cdot x + 2 x \cdot - 2 + 2 \cdot (2 x) + 2 \cdot 2 \cdot - 2}$

Step 8.2.5

Move $x$ .

$\frac{x^{3}}{2 x \cdot x + 2 \cdot (- 2 x) + 2 \cdot (2 x) + 2 \cdot 2 \cdot - 2}$

Step 8.2.6

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

$\frac{x^{3}}{2 (x \cdot x) + 2 \cdot (- 2 x) + 2 \cdot (2 x) + 2 \cdot 2 \cdot - 2}$

Step 8.2.7

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

$\frac{x^{3}}{2 (x \cdot x) + 2 \cdot (- 2 x) + 2 \cdot (2 x) + 2 \cdot 2 \cdot - 2}$

Step 8.2.8

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

$\frac{x^{3}}{2 x^{1 + 1} + 2 \cdot (- 2 x) + 2 \cdot (2 x) + 2 \cdot 2 \cdot - 2}$

Step 8.2.9

Add $1$ and $1$ .

$\frac{x^{3}}{2 x^{2} + 2 \cdot (- 2 x) + 2 \cdot (2 x) + 2 \cdot 2 \cdot - 2}$

Step 8.2.10

Multiply $2$ by $- 2$ .

$\frac{x^{3}}{2 x^{2} - 4 x + 2 \cdot (2 x) + 2 \cdot 2 \cdot - 2}$

Step 8.2.11

Multiply $2$ by $2$ .

$\frac{x^{3}}{2 x^{2} - 4 x + 4 x + 2 \cdot 2 \cdot - 2}$

Step 8.2.12

Multiply $2$ by $2$ .

$\frac{x^{3}}{2 x^{2} - 4 x + 4 x + 4 \cdot - 2}$

Step 8.2.13

Multiply $4$ by $- 2$ .

$\frac{x^{3}}{2 x^{2} - 4 x + 4 x - 8}$

Step 8.2.14

Add $- 4 x$ and $4 x$ .

$\frac{x^{3}}{2 x^{2} + 0 - 8}$

Step 8.2.15

Subtract $8$ from $0$ .

$\frac{x^{3}}{2 x^{2} - 8}$

Step 8.3

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

$2 x^{2}$

$0 x$

$8$

$x^{3}$

$0 x^{2}$

$0 x$

$0$

Step 8.4

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

$\frac{x}{2}$

$2 x^{2}$

$0 x$

$8$

$x^{3}$

$0 x^{2}$

$0 x$

$0$

Step 8.5

Multiply the new quotient term by the divisor.

$\frac{x}{2}$

$2 x^{2}$

$0 x$

$8$

$x^{3}$

$0 x^{2}$

$0 x$

$0$

$x^{3}$

$0$

$4 x$

Step 8.6

The expression needs to be subtracted from the dividend, so change all the signs in $x^{3} + 0 - 4 x$

$\frac{x}{2}$

$2 x^{2}$

$0 x$

$8$

$x^{3}$

$0 x^{2}$

$0 x$

$0$

$x^{3}$

$0$

$4 x$

Step 8.7

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

$\frac{x}{2}$

$2 x^{2}$

$0 x$

$8$

$x^{3}$

$0 x^{2}$

$0 x$

$0$

$x^{3}$

$0$

$4 x$

Step 8.8

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

$\frac{x}{2}$

$2 x^{2}$

$0 x$

$8$

$x^{3}$

$0 x^{2}$

$0 x$

$0$

$x^{3}$

$0$

$4 x$

$0$

Step 8.9

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

$\frac{x}{2} + \frac{4 x}{2 x^{2} - 8}$

Step 8.10

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

$y = \frac{x}{2}$

Step 9

This is the set of all asymptotes.

Vertical Asymptotes: $x = - 2, 2$

No Horizontal Asymptotes

Oblique Asymptotes: $y = \frac{x}{2}$

Step 10