Finding Vertical Asymptotes

# FINDING VERTICAL ASYMPTOTES

• PRACTICE (online exercises and printable worksheets)

As introduced in the earlier section, Introduction to Asymptotes:

DEFINITION asymptote
An ‘asymptote’ (pronounced AS-sim-tote) is a curve (usually a line)
that another curve gets arbitrarily close to as $\,x\,$ approaches $\,+\infty\,$, $\,-\infty\,$, or a finite number.

In particular, here is the definition of a vertical asymptote:

 DEFINITION vertical asymptote A ‘vertical asymptote’ is a vertical line that another curve gets arbitrarily close to as $\,x\,$ approaches a finite number. Specifically, the vertical line $\,x = c\,$ is a vertical asymptote for a function $\,f\,$ if and only if at least one of the following conditions is true: as $\,x\rightarrow c^+\,$, $\,f(x)\rightarrow\pm\infty\,$ as $\,x\rightarrow c^-\,$, $\,f(x)\rightarrow\pm\infty\,$ The tangent function has infinitely many vertical asymptotes. For example,the vertical line $\,x = \frac{\pi}{2}\,$ is a vertical asymptote. as $\,x\rightarrow {\frac{\pi}{2}}^{+}\,$, $\,\tan x\rightarrow -\infty\,$ as $\,x\rightarrow {\frac{\pi}{2}}^{-}\,$, $\,\tan x\rightarrow \infty\,$

• Roughly, outputs ‘blow up’ near a vertical asymptote!
They get very big and positive (go to $\,+\infty\,$), or very big and negative (go to $\,-\infty\,$).
(A ‘big’ number is far from zero—it can be to the right or left of zero.)
Typically, the closer the inputs to the value of $\,x\,$ where the vertical asymptote occurs, the bigger the outputs.
• The sentence ‘$\,x\rightarrow c^+\,$’ is read aloud as ‘$\,x\,$ approaches $\,c\,$ from the right’.
This is made precise in Calculus, but roughly means that $\,x\,$ is greater than $\,c\,$ and getting as close to $\,c\,$ as desired from this right-hand side.
• Similarly, ‘$\,x\rightarrow c^-\,$’ is read aloud as ‘$\,x\,$ approaches $\,c\,$ from the left’.
Roughly, this means that $\,x\,$ is less than $\,c\,$ and getting as close to $\,c\,$ as desired from this left-hand side.
• ALL THESE GRAPHS HAVE VERTICAL ASYMPTOTE  $\,\color{red}{x = 2}\,$ :
 as $\,x\rightarrow 2^+\,$, $\,y \rightarrow \infty$ as $\,x\rightarrow 2^+\,$, $\,y \rightarrow -\infty$ as $\,x\rightarrow 2^-\,$, $\,y \rightarrow \infty$ as $\,x\rightarrow 2^-\,$, $\,y \rightarrow -\infty$ as $\,x\rightarrow 2^+\,$, $\,y \rightarrow \infty$ as $\,x\rightarrow 2^-\,$, $\,y \rightarrow \infty$ as $\,x\,$ approaches $\,2\,$from the right,$\,y\,$ goes to infinity as $\,x\,$ approaches $\,2\,$from the right,$\,y\,$ goes tonegative infinity as $\,x\,$ approaches $\,2\,$from the left,$\,y\,$ goes to infinity as $\,x\,$ approaches $\,2\,$from the left,$\,y\,$ goes tonegative infinity

## VERTICAL ASYMPTOTES FOR RATIONAL FUNCTIONS

How can a rational function (a ratio of polynomials) ‘blow up’ near a finite input?
When you're ‘trying’ to divide by zero!
Remember—dividing by a very small number gives a very big number.

VERTICAL ASYMPTOTES for RATIONAL FUNCTIONS
A vertical asymptote for a rational function occurs where the denominator is zero,
and the numerator is nonzero.

In other words, for polynomials $\,N\,$ and $\,D\,$, the rational function $\displaystyle \,\frac{N(x)}{D(x)}\,$
has a vertical asymptote at $\,c\,$ wherever $\,D(c) = 0\,$ and $\,N(c)\ne 0\,$.
• A rational function might not have any vertical asymptotes.
For example, $\,\displaystyle f(x) = \frac{1}{x^2 + 1}\,$ has no vertical asymptotes, because the denominator is never zero.
• The condition $\,N(c)\ne 0\,$ is important.

If $\,N(c) = 0\,$ (as in the following two examples), you could have either a vertical asymptote or a hole at $\,c\,$:
• The function $\displaystyle\,\frac{x-2}{(x-2)^2}\,$ has a vertical asymptote at $\,2\,$.     Why? $\displaystyle\,\frac{x-2}{(x-2)^2} = \frac{1}{x-2}$

In this case, there are more factors of $\,x-2\,$ in the denominator than in the numerator.
• The function $\displaystyle\,\frac{(x-2)^2}{x-2}\,$ has a hole at $\,2\,$.     Why?   For $\,x\ne 2\,$, $\displaystyle\,\frac{(x-2)^2}{x-2} = x-2\,$.

In this case, there are more factors of $\,x-2\,$ in the numerator than in the denominator.

## CHECKING THE BEHAVIOR OF A RATIONAL FUNCTION NEAR A VERTICAL ASYMPTOTE

Given a vertical asymptote, you usually want to know how the function behaves nearby.
Are the outputs going to infinity? To negative infinity?
Although you could certainly use a graphing calculator or WolframAlpha to see this behavior,
you should also be able to determine it algebraically, as shown next.

EXAMPLE:
The function $\displaystyle\,f(x) = \frac{-3}{2x + 1}\,$ has a vertical asymptote at $\,x = -\frac 12\,$.

Why?   The denominator, $\,2x + 1\,$, is zero when $\,x = -\frac 12\,$, and the numerator is (always) nonzero.

Here is the thought process for determining what the outputs from $\,f\,$ look like, close to $\,-\frac 12\,$:
 Consider values of $\,x\,$ a bit less than $\,-\frac 12\,$ (i.e., just to the left of $\,-\frac 12\,$): the denominator is small and negative (see the graph of $\,y = 2x+1\,$) the numerator is negative Thus:   as $\,x\rightarrow {-\frac 12}^{-}\,$, $\,f(x) \approx \frac{\color{green}{(-)}}{\color{red}{(\text{small }-)}} \rightarrow +\infty\,$. Observe that the notation   $\,\frac{(-)}{(\text{small }-)}\,$   is being used to denote a (normal-sized) negative number, divided by a small negative number—which produces a large positive number. Next, consider values of $\,x\,$ a bit more than $\,-\frac 12\,$ (i.e., just to the right of $\,-\frac 12\,$): the denominator is small and positive (see the graph of $\,y = 2x+1\,$) the numerator is negative Thus:   as $\,x\rightarrow {-\frac 12}^{+}\,$, $\,f(x) \approx \frac{\color{green}{(-)}}{\color{blue}{(\text{small }+)}} \rightarrow -\infty\,$. Observe that we did not compute actual values of the output near $\,x = -\frac 12\,$! There's no need—it would be working too hard. All we need to know is: the sign (positive or negative) of the numerator and denominator the denominator is small (close to zero) dividing by increasingly small numbers give increasingly big numbers

Master the ideas from this section