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How to Interpret and Calculate Asymptotes of a Function

Asymptotes are lines that the graph approaches, but never meets. Asymptotes are imaginary lines that you want to draw as dashed lines, so it’s easy to see where they are while indicating that they aren’t part of the graph.


Vertical Asymptotes

The line x = a is a vertical asymptote if f(x) ± when x a.

Vertical asymptotes occur when the denominator of a fraction is zero, because the function is undefined there.


Horizontal asymptotes

The line y = c is a horizontal asymptote if f(x) c when x ±.

Note! There are also oblique (slanted) asymptotes, but they are not as common.

Example 1

You have the expression f(x) = x2+2x+1 x2 . Find any vertical and horizontal asymptotes.

Vertical Asymptotes

To find the vertical asymptotes, you need to set the denominator equal to zero:

x 2 = 0 x = 2

Horizontal Asymptotes

You find the horizontal asymptotes by calculating the limit: lim xx2 + 2x + 1 x 2 = lim xx2 x2 + 2x x2 + 1 x2 x x2 2 x2 = lim x1 + 2 x + 1 x2 1 x 2 x = 1 + 0 + 0 0 divergent

Note! The word “divergent” in this context means that the limit does not exist.

The figure shows the graph of the function in Example 1. As you can see, there is a vertical asymptote at x = 2 and no horizontal asymptote. There is an oblique asymptote.

Example of vertical asymptote

Example of vertical asymptote

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