Back to homeMathematics · AP Calculus AB · Change subject
Start for free
AP Calc AB · Mathematics
Limits and Continuity · Limits

Asymptotes of a Function

Before you start, make sure you know: Calculation of limits (Precalculus)
Before you start, make sure you know: Limit of a function as x → ∞ (Precalculus)

You pour a cup of hot cocoa at 190 °F in a 70 °F kitchen. It cools fast at first, then more and more slowly, and it gets closer and closer to 70 °F without ever quite reaching it. A line that a graph gets closer and closer to is called an asymptote. In this page you will learn to find all three kinds with limits, and to tell which side of the line the curve is on.

Limits you need first

Asymptotes are described with limits, so let's make sure the toolkit is ready.

Graph with a jump at x = c: the left piece approaches L1 and the right piece approaches a different value L2, so the limit does not exist. x = c x → c⁻ : f(x) → L₁ x → c⁺ : f(x) → L₂ L₁ ≠ L₂, so the limit DNE

Direct substitution. For polynomials, rational functions (when the denominator is not $0$), $e^x$, $\sin x$ and similar functions, $\displaystyle\lim_{x\to c}f(x)=f(c)$. For example, $\displaystyle\lim_{x\to -1}\frac{x^2+4}{x+3}=\frac{5}{2}$.

When substitution gives $\tfrac00$, that is an indeterminate form, not an answer. The function can still have a limit, so simplify first: factor and cancel, or multiply by a conjugate. For $x^3-8$ use $a^3-b^3=(a-b)(a^2+ab+b^2)$, or divide by $x-2$ with synthetic division.

Practice★☆☆Not started
Observe the following graph representing the RAM memory consumption (in GB) of a process server as a function of time $t$ (in seconds). Determine, intuitively, the value…

Observe the following graph representing the RAM memory consumption (in GB) of a process server as a function of time $t$ (in seconds). Determine, intuitively, the value of the one-sided limits at the instant $t = 4$ and reason whether the overall limit exists at that point.

1 2 3 4 5 6 7 8 1 2 3 4 5 Time t (s) RAM memory (GB) t → 4⁻ t → 4⁺
Solve
Practice★☆☆Not started
Calculate the following direct limits by substitution, which represent the velocity of a particle in feet per second at different instants $t$: a)…

Calculate the following direct limits by substitution, which represent the velocity of a particle in feet per second at different instants $t$:

  1. a) $\lim_{t \to 2} (t^2 + 3t - 1)$
  1. b) $\lim_{t \to 0} \frac{e^t + 1}{t + 2}$
Solve
Practice★★☆Not started
Calculate the following limits by resolving the corresponding indeterminate forms: a) $\displaystyle \lim_{x \to 0} \frac{\sqrt{1+x} - \sqrt{1-x}}{x}$ b)…

Calculate the following limits by resolving the corresponding indeterminate forms:

  1. a) $\displaystyle \lim_{x \to 0} \frac{\sqrt{1+x} - \sqrt{1-x}}{x}$
  1. b) $\displaystyle \lim_{x \to 2} \frac{x^3 - 8}{x^2 - 4}$
Solve

Vertical asymptotes

Graph of a function whose two branches both shoot up to positive infinity on either side of the vertical asymptote x = c. x = c both sides shoot up, like 1/(x − c)²

To decide $+\infty$ or $-\infty$, check the sign of the fraction just to the left and just to the right of $c$.

Practice★☆☆Not started
Given the rational function $f(x) = \frac{5}{x - 2}$, which describes the intensity of a magnetic field in relation to the distance $x$ to a conducting wire: a)…

Given the rational function $f(x) = \frac{5}{x - 2}$, which describes the intensity of a magnetic field in relation to the distance $x$ to a conducting wire:

  1. a) Determine the domain of the function.
  2. b) Calculate the one-sided limits $\lim_{x \to 2^-} f(x)$ and $\lim_{x \to 2^+} f(x)$.
  3. c) State the equation of its vertical asymptote.
x y 0 2 x = ? f(x) x → 2⁻ f(x) → ? x → 2⁺ f(x) → ? f(x) = 5/(x-2) Dom(f) = ?
Solve
Practice★☆☆Not started
A technical inspection drone follows an approach trajectory to an antenna whose height is given by the function $H(x) = \frac{x^2 - 9}{x - 3}$, where $x$ is the…

A technical inspection drone follows an approach trajectory to an antenna whose height is given by the function $H(x) = \frac{x^2 - 9}{x - 3}$, where $x$ is the horizontal distance to the control center. Calculate the limit of the height when the drone is very close to $x = 3$ feet.

(Note: Although the function is not defined at $x=3$, study the limit by simplifying the expression).

Solve
Practice★★☆Not started
Given the function $f(x) = \dfrac{x^2 - k}{x^2 - 4x + 4}$: a) Find the value of $k$ so that the function has a removable discontinuity at $x = 2$. b) If $k = 9$, study…

Given the function $f(x) = \dfrac{x^2 - k}{x^2 - 4x + 4}$:

  1. a) Find the value of $k$ so that the function has a removable discontinuity at $x = 2$.
  2. b) If $k = 9$, study the existence of vertical asymptotes and describe the behavior of the function in their vicinity using one-sided limits.
0 x = 2 y = 1 ? ? k=? f(x)= (x²-k)/(x-2)² a) k=? (removable) b) k=9, lim = ?
Solve

End behavior of polynomials

What happens to a function as $x\to\pm\infty$ is called end behavior. We start with polynomials, because the same idea (the biggest power wins) powers everything else.

Practice★☆☆Not started
Determine the behavior at infinity of the following polynomial functions, indicating whether they tend to $+\infty$ or $-\infty$: a) $f(x) = -2x^3 + 5x - 1$ as…

Determine the behavior at infinity of the following polynomial functions, indicating whether they tend to $+\infty$ or $-\infty$:

  1. a) $f(x) = -2x^3 + 5x - 1$ as $x \to +\infty$
  1. b) $g(x) = x^4 - 100x^2$ as $x \to -\infty$
Solve

Horizontal asymptotes

For a rational function $\dfrac{P(x)}{Q(x)}$, compare degrees:

Degrees Limit at $\pm\infty$ Horizontal asymptote
$\deg P<\deg Q$ $0$ $y=0$
$\deg P=\deg Q$ ratio of leading coefficients $y=\dfrac{a_n}{b_n}$
$\deg P>\deg Q$ $\pm\infty$ none (if exactly one more, there is an oblique asymptote)

Why? Divide every term by the highest power of $x$ in the denominator. Every leftover $\frac1{x^k}$ goes to $0$, and only the leading coefficients survive.

Practice★☆☆Not started
In a biological study, the number of individuals in a population of bacteria (in thousands) follows the model $P(t) = \frac{4t + 10}{t + 2}$, where $t$ is the time in…

In a biological study, the number of individuals in a population of bacteria (in thousands) follows the model $P(t) = \frac{4t + 10}{t + 2}$, where $t$ is the time in hours.

  1. a) Calculate $\lim_{t \to +\infty} P(t)$ to find the long-term population.
  2. b) What type of asymptote does this result represent? Write its equation.
Solve
Practice★☆☆Not started
Given the function $f(x) = \frac{x^2}{x^2 + 4}$, which models the efficiency of a photovoltaic panel as a function of the received radiation $x$, justify by calculating…

Given the function $f(x) = \frac{x^2}{x^2 + 4}$, which models the efficiency of a photovoltaic panel as a function of the received radiation $x$, justify by calculating limits whether the function has any horizontal asymptote and represent it graphically in a schematic way.

y = 1 limₓ→±∞ f(x) = 1 −8 −6 −4 −2 2 4 6 8 0.5 1.0 0 Radiation (x) Efficiency f(x) f(x)
Solve
Practice★★☆Not started
A team of engineers is monitoring the temperature $T$ (in degrees Fahrenheit) of a new graphics processor as a function of the time $t$ (in minutes) since it was turned…

A team of engineers is monitoring the temperature $T$ (in degrees Fahrenheit) of a new graphics processor as a function of the time $t$ (in minutes) since it was turned on. The function is given by:

$$T(t) = \frac{80t^2 + 150}{2t^2 + 3}$$

Calculate the initial temperature of the processor and determine, by calculating limits, whether the temperature stabilises in the long term. What type of asymptote does this stabilisation value represent?

Solve

Oblique asymptotes

The leftover term $\dfrac{R(x)}{Q(x)}$ also tells you the position of the curve: where it is positive the curve is above the line, where it is negative the curve is below.

Graph of a function with a vertical asymptote and an oblique asymptote; the right branch is above the slanted line and the left branch is below it. x = c oblique asymptote y = mx + b

Absolute values. (Some problems call these modulus functions.) The ends of the graph can follow different lines, because $|x+1|$ equals $x+1$ on the right end and $-(x+1)$ on the left end. Treat each end separately.

Practice★★☆Not started
Find the equation of the oblique asymptote of the function $f(x) = \dfrac{2x^2 - 3x + 1}{x - 2}$. To complete the study, indicate the relative position of the curve with…

Find the equation of the oblique asymptote of the function $f(x) = \dfrac{2x^2 - 3x + 1}{x - 2}$. To complete the study, indicate the relative position of the curve with respect to this asymptote as $x \to +\infty$ and as $x \to -\infty$.

Solve
Practice★★★Not started
Consider the function $f(x)$ defined using modulus functions to model the trajectory of a surveillance drone: $$f(x) = \frac{x \cdot |x-2|}{x+1}$$ Study the existence of…

Consider the function $f(x)$ defined using modulus functions to model the trajectory of a surveillance drone:

$$f(x) = \frac{x \cdot |x-2|}{x+1}$$

Study the existence of oblique asymptotes for this function. If they exist, calculate their equations and analyse the position of the curve with respect to these asymptotes at both $+\infty$ and $-\infty$.

x y 5 10 −5 −10 5 8 x = −1 y = x - 3 (x → +∞) y = -x + 3 (x → -∞) f(x) Curve above Curve above
Solve
Practice★★★Not started
Given the function dependent on the parameters $k$ and $m$: $$f(x) = \frac{kx^3 + mx^2 + 1}{(x-1)^2}$$ Calculate the values of $k$ and $m$ so that $f(x)$ has an oblique…

Given the function dependent on the parameters $k$ and $m$:

$$f(x) = \frac{kx^3 + mx^2 + 1}{(x-1)^2}$$

Calculate the values of $k$ and $m$ so that $f(x)$ has an oblique asymptote passing through the point $(0, 2)$ and whose slope is $3$. Once found, check if the function has any vertical asymptote and justify whether it is removable or not.

Solve

Radicals at infinity

A radical can produce the form $\infty-\infty$. Multiply by the conjugate to get a fraction, then divide by the highest power of $x$.

The key fact is $\sqrt{x^2}=|x|$. For $x\to+\infty$ this is $x$, but for $x\to-\infty$ it is $-x$. That is why the two ends can behave very differently.

Practice★★☆Not started
Consider the function $f(x) = \sqrt{x^2 + 4x} - x$. a) Calculate $\displaystyle \lim_{x \to +\infty} f(x)$. b) Check if there is a horizontal asymptote as $x$ tends to…

Consider the function $f(x) = \sqrt{x^2 + 4x} - x$.

  1. a) Calculate $\displaystyle \lim_{x \to +\infty} f(x)$.
  2. b) Check if there is a horizontal asymptote as $x$ tends to $+\infty$.
  3. c) Investigate what happens to the limit of the function as $x \to -\infty$. Is there any asymptote on that branch?
Solve

Putting it together: all the asymptotes

To describe a rational function completely, follow this checklist:

  1. Factor numerator and denominator, and cancel common factors (holes).

  2. Zeros of the remaining denominator are vertical asymptotes. Use signs to find each one-sided limit.

  3. Compare degrees for the end behavior: horizontal asymptote or oblique asymptote.

Reading asymptotes from a graph. Look for dashed or "escaping" behavior. If the curve rises or falls without bound near $x=c$, then $x=c$ is a vertical asymptote and you read the one-sided limits from the direction of the branches. If the curve flattens toward a height $L$ at the far left or right, then $y=L$ is a horizontal asymptote on that side.

Graph with a vertical asymptote x = c and two different horizontal asymptotes y = L1 on the left and y = L2 on the right. y = L₁ y = L₂ x = c x → c⁺ : −∞ x → c⁻ : +∞
Practice★☆☆Not started
Find all the asymptotes (vertical and horizontal) of the function $f(x) = \frac{3x - 6}{x + 1}$. To do this: 1. Identify the value of $x$ that makes the denominator…

Find all the asymptotes (vertical and horizontal) of the function $f(x) = \frac{3x - 6}{x + 1}$. To do this:

  1. Identify the value of $x$ that makes the denominator zero.

  2. Calculate the limit of the function as $x \to \infty$.

Solve
Practice★★☆Not started
Observe the following graph of a function $f(x)$ and determine, from the visual information, the value of the following limits and the equations of all its asymptotes:…

Observe the following graph of a function $f(x)$ and determine, from the visual information, the value of the following limits and the equations of all its asymptotes:

  1. a) $\displaystyle \lim_{x \to -\infty} f(x)$
  2. b) $\displaystyle \lim_{x \to 1^-} f(x)$ and $\displaystyle \lim_{x \to 1^+} f(x)$
  3. c) $\displaystyle \lim_{x \to +\infty} f(x)$
x y f(x) 0 1 2 y = 0 x = 1 y = 2
Solve

The $1^\infty$ form and the number $e$

If the base tends to $1$ and the exponent tends to $\infty$, the limit is not automatically $1$. Remember

$$\lim_{u\to\infty}\left(1+\frac1u\right)^u=e\approx2.718$$
Practice★★☆Not started
Calculate the value of the following limit, which results in an indeterminate form of the type $1^\infty$, using the relationship with the number $e$: $$\lim_{x \to…

Calculate the value of the following limit, which results in an indeterminate form of the type $1^\infty$, using the relationship with the number $e$:

$$\lim_{x \to +\infty} \left( \frac{3x + 5}{3x - 2} \right)^{2x + 1}$$
Solve
Practice★★★Not started
A new data compression algorithm is being designed for a streaming platform. The response time of the system, in milliseconds, is modelled by the function $T(x)$, where…

A new data compression algorithm is being designed for a streaming platform. The response time of the system, in milliseconds, is modelled by the function $T(x)$, where $x$ represents the workload in terabytes:

$$T(x) = \left( \frac{x^2 + ax + 1}{x^2 + 3} \right)^{2x+1}$$

Knowing that, for the system to be stable under extreme loads, the limit of the response time when the load tends to infinity must be exactly $e^4$, determine with reasoning the value of the parameter $a$.

Solve

Growth rates and logarithms

When a limit at infinity has $\frac{\infty}{\infty}$, you need to know which part grows faster. For large $x$ (and $b>1$, $p>0$):

$$\ln x\ \ll\ x^p\ \ll\ b^x$$

Exponentials beat powers, and powers beat logarithms. (Some problems call this comparing growth rates the hierarchy of infinities.) So $\displaystyle\lim_{x\to\infty}\frac{x^{p}}{b^x}=0$ and $\displaystyle\lim_{x\to\infty}\frac{\ln x}{x^{p}}=0$.

The strategy is to factor out the dominant term inside the expression, then use the ranking to show the rest tends to $0$.

Logarithms and vertical asymptotes. $\ln u$ is only defined for $u>0$. Near a finite $x=c$, $\ln u\to-\infty$ if $u\to0^+$ and $\ln u\to+\infty$ if $u\to+\infty$. So vertical asymptotes appear at the finite endpoints of the domain where the argument tends to $0$ or $\infty$.

Practice★★★Not started
In the study of the spread of a computer virus on a social network, it has been determined that the number of infected nodes $N(t)$ after a time $t$ (in hours) follows…

In the study of the spread of a computer virus on a social network, it has been determined that the number of infected nodes $N(t)$ after a time $t$ (in hours) follows the function:

$$N(t) = \frac{ln(e^{3t} + t^2)}{t + \sqrt{t^2 + 1}}$$

Analyse the long-term behavior of the infection by calculating $\lim_{t \to +\infty} N(t)$. Does the function present any horizontal asymptote? Justify your answer by applying the corresponding hierarchies of infinities.

Solve
Practice★★★Not started
Study the domain and all the asymptotes of the following logarithmic function: $$f(x) = ln\left( \frac{x^2-4}{x^2-1} \right)$$ Determine the one-sided limits at each of…

Study the domain and all the asymptotes of the following logarithmic function:

$$f(x) = ln\left( \frac{x^2-4}{x^2-1} \right)$$

Determine the one-sided limits at each of the points where the function is not defined and sketch the behavior of the function near its vertical asymptotes.

? ? ? ? Undefined Undefined x y −2 −1 1 2 0 x=? x=? x=? x=? y=? f(x)=ln x²-4 x²-1
Solve

L'Hôpital's Rule

For $\infty-\infty$, combine into a single fraction first so you get $\frac00$.

Practice★★★Not started
Calculate the value of the following limit using L'Hôpital's Rule after making the necessary transformations to resolve the indeterminate form: $$\lim_{x \to 0} \left(…

Calculate the value of the following limit using L'Hôpital's Rule after making the necessary transformations to resolve the indeterminate form:

$$\lim_{x \to 0} \left( \frac{1}{x^2} - \frac{1}{x \cdot sen(x)} \right)$$

From the result obtained, indicate whether the function $f(x) = \frac{sen(x) - x}{x^2 \cdot sen(x)}$ has a vertical asymptote at the origin.

Solve

Solving for parameters

Many problems give you the asymptote and ask for an unknown constant. The idea is always the same: write the limit with the unknown, decide what must happen for the limit to be finite, and then set it equal to the required value.

Piecewise functions. Each piece has its own job: the left piece controls the limit as $x\to-\infty$ and the one-sided limit at $c$ from the left, while the right piece controls $+\infty$ and the limit from the right. For a piece of the form $\dfrac{\text{stuff}}{x-c}$ to have a finite one-sided limit at $c$, the numerator must be $0$ at $c$, so that you get $\frac00$ and can cancel.

Practice★★☆Not started
Given the rational function $f(x) = \dfrac{ax^2 + 5x - 3}{2x^2 - 8}$, determine the value of the real parameter $a$ knowing that the function has a horizontal asymptote…

Given the rational function $f(x) = \dfrac{ax^2 + 5x - 3}{2x^2 - 8}$, determine the value of the real parameter $a$ knowing that the function has a horizontal asymptote at the line $y = 3$. Once the value of $a$ is found, calculate the one-sided limits of the function at its vertical asymptotes and explain the behavior of the graph at those points.

x y 0 −2 2 3 y = 3 x = −2 x = 2 f(x) x → −2⁻ x → −2⁺ x → 2⁻ x → 2⁺
Solve
Practice★★★Not started
A communications satellite follows an orbit whose gravitational perturbation is described by the function $g(x)$. It is required that the satellite does not move…

A communications satellite follows an orbit whose gravitational perturbation is described by the function $g(x)$. It is required that the satellite does not move indefinitely away from a planned linear trajectory. The function is:

$$g(x) = \sqrt{4x^2 + bx - 3} - ax$$

Find the values of the real parameters $a$ and $b$ so that the function has a horizontal asymptote at $y = 2$ when $x \to +\infty$.

Solve
Practice★★★Not started
Let $f(x)$ be the piecewise function: $$f(x) = \begin{cases} \frac{x^2 - k}{x-2} & \text{if } x < 2 \\ \sqrt{x^2 + ax} - x & \text{if } x \geq 2 \end{cases}$$ 1. Find…

Let $f(x)$ be the piecewise function:

$$f(x) = \begin{cases} \frac{x^2 - k}{x-2} & \text{if } x < 2 \\ \sqrt{x^2 + ax} - x & \text{if } x \geq 2 \end{cases}$$
  1. Find the value of $k$ so that the function has a removable discontinuity at $x = 2$.

  2. For the value of $k$ obtained in the previous section, determine the value of $a$ so that the function has a horizontal asymptote $y = 3$ as $x \to +\infty$.

Solve

Summary

See all the exercises in this section