Asymptotes

In this lesson, we will look into how to find vertical asymptotes, horizontal asymptotes and oblique asymptotes.

Vertical Asymptote

How to determine the Vertical Asymptote?

Method 1: Use the Definition of Vertical Asymptote

The line x = a is called a Vertical Asymptote of the curve y = f(x) if at least one of the following statements is true.

Method 2:

For rational functions f(x)

Step 1: Write f(x) in reduced form

Step 2: if x – c is a factor in the denominator then x = c is the vertical asymptote.

Example:

Find the vertical asymptotes of

Solution:

Method 1: Use the definition of Vertical Asymptote.

If x is close to 3 but larger than 3, then the denominator x – 3 is a small positive number and 2x is close to 8. So, is a large positive number.

Intuitively, we see that

Similarly, if x is close to 3 but smaller than 3, then x – 3 is a small negative number and 2x is close to 8. So, is a large negative number.

The line x = 3 is the vertical asymptote.

Method 2:

Step 1: f(x) is already in reduced form.

Step 2: The denominator is x – 3 and so the Vertical Asymptote is at x = 3.

Horizontal Asymptote

How to determine the horizontal Asymptote?

Method 1: Use the definition of Horizontal Asymptote

The line y = L is called a horizontal asymptote of the curve y = f(x) if either

Method 2:

For rational functions f(x)

If the degree of x in the numerator is less than the degree of x in the denominator then y = 0 is the horizontal asymptote.

If the degree of x in the numerator is equal to the degree of x in the denominator then y = c where c is obtained by dividing the leading coefficients.

Example:

Find the horizontal and vertical asymptotes of the function

Solution:

Method 1:

Divide both numerator and denominator by x.

The line is the horizontal asymptote.

Method 2:

The degree of x in the numerator is equal to the degree of x in the denominator.

Dividing the leading coefficients we get

The line is the horizontal asymptote.

Oblique Asymptote or Slant Asymptote

Some curves have asymptotes that are oblique, that is, neither horizontal nor vertical.

If then the line y = mx + b is called the oblique or slant asymptote because the vertical distances between the curve y = f(x) and the line y = mx + b approaches 0.

For rational functions, oblique asymptotes occur when the degree of the numerator is one more than the degree of the denominator. In such a case the equation of the oblique asymptote can be found by long division.

Example:

Find the asymptotes of the function

Solution:

Since the denominator x² + 1 is never 0, there is no vertical asymptote.

Since the degree of x in the numerator is greater than the degree of x in the denominator there is no horizontal asymptote.

Since the degree of x in the numerator is one greater than the degree of x in the denominator we can use long division to obtain the oblique asymptote.

So, the line y = x is the oblique asymptote.

Videos

Finding Vertical Asymptotes of Rational Functions
What to look for, in order to find vertical asymptotes of rational functions

Shortcut to Find Horizontal Asymptotes of Rational Functions
A couple of tricks that make finding horizontal asymptotes of rational functions very easy to do

Finding Slant Asymptotes of Rational Functions
When a rational function has a slant asymptote, briefly describe what a slant asymptote

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