Asymptote (geometry)

Revision as of 21:36, 9 November 2007 by Azjps (talk | contribs) (Horizontal Asymptotes: expand; example not very suitable)
This is an AoPSWiki Word of the Week for Nov 8-14
For the vector graphics language, see Asymptote (Vector Graphics Language).

An asymptote is a line or curve that a certain function approaches.

Asymptotes can be of three different kinds: horizontal, vertical or slanted (oblique).

Vertical Asymptotes

The vertical asymptote can be found by finding values of $x$ that make the function undefined. One of the common ways is to have the function divided by zero, which is undefined. This can be shown by example.

Example Problem

Find the vertical asymptotes of $\frac{1}{x^{2}}$.


To find the vertical asymptotes, $x^2$ must equal zero. Solving the equation:

$\begin{eqnarray*}x^2&=&0\\x&=&\boxed{0}\end{eqnarray*}$ (Error compiling LaTeX. ! Missing \endgroup inserted.)

So the vertical asymptote is $x=0$, or just the y-axis

Horizontal Asymptotes

The horizontal asymptote can be found in the same method as vertical asymptotes, but in relation to $y$ instead of $x$.

In general, to find a horizontal asymptote, take the $\lim_{x \rightarrow \infty} f(x)$ and $\lim_{x \rightarrow -\infty} f(x)$ to find the end behavior of the function. For rational functions in the form of $\frac{P(x)}{Q(x)}$ where $P(x), Q(x)$ are both polynomials, if the degree of the $Q(x)$ is greater than that of the degree of $P(x)$, then the horizontal asymptote is at $y = 0$. If the degree of $Q(x)$ is equal to that of the degree of $P(x)$, then the horizontal asymptote is at the quotient of the leading coefficient of $P(x)$ over the leading coefficient of $Q(x)$. (If the degree of $Q(x)$ is less than that of $P(x)$, then you get a slant asymptote, explained in the next section).

Note a crucial difference between horizontal asymptotes and vertical asymptotes: a function can never be defined at a vertical asymptote, but it can be defined at a horizontal asymptote. This is because the function is undefined (division by zero) at vertical asymptotes. However, a horizontal asymptote only gives the values for the ends of the function, but doesn’t have anything to do with the behavior of the function in the “middle”.

Horizontal asymptotes also occur in the inverses of certain functions with vertical asymptotes, and can occur in rotated conics, namely hyperbolas. For example, the hyperbola $xy = 1$ has a horizontal asymptote at $y = 0$.

Example Problem

Find the horizontal asymptote of $f(x) = \frac{x^2 - 3x + 2}{-2x^2 + 15x + 10000}$.


If we take $\lim_{x \rightarrow \pm\infty} f(x)$, notice that the $x^2$ term grows at a faster rate than the rest of the terms; hence our answer is $-\frac{1}{2}$.

Slanted Asymptotes

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