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Limits and continuity · Limits

Asymptotes of a Function

Before you start, make sure you know: Calculating limits of functions (High School Year 2 (semesters 3 and 4))
Before you start, make sure you know: Limit of a function as x → ∞ (High School Year 2 (semesters 3 and 4))

During El Buen Fin, an online store gets more and more visitors. Does the time it takes to respond keep growing without bound, or does it level off at some value? And what happens right when the store reaches its capacity limit? Asymptotes are the lines that answer these questions: they describe where the graph is heading when it "escapes."

Review: limits we are going to need

To find asymptotes, all you have to do is calculate limits. These are the basic tools:

  • Direct substitution. If the function is continuous at $x_0$, then $\lim_{x\to x_0} f(x)=f(x_0)$. For example, $\lim_{t\to 2}(t^2+3t-1)=4+6-1=9$.

  • One-sided limits. $\lim_{x\to x_0^-}$ (from the left) and $\lim_{x\to x_0^+}$ (from the right). The limit exists only if both agree. If, in a graph, the curve approaches different heights from each side, the overall limit does not exist.

  • Polynomials at infinity. The highest-degree term dominates. For example, $-2x^3+5x-1\to-\infty$ as $x\to+\infty$, because $-2x^3$ is negative for large $x$. And $x^4-100x^2\to+\infty$ as $x\to-\infty$, because $x^4$ is always positive and beats $100x^2$.

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Determine the behavior at infinity of the following polynomial functions, stating 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, stating 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
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Calculate the following direct limits by substitution, representing the velocity of a particle in meters per second at different times $t$: a)…

Calculate the following direct limits by substitution, representing the velocity of a particle in meters per second at different times $t$:

  1. a) $\lim_{t \to 2} (t^2 + 3t - 1)$
  1. b) $\lim_{t \to 0} \frac{e^t + 1}{t + 2}$
Solve
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Observe the following graph representing the RAM memory usage (in GB) of a process server as a function of time $t$ (in seconds). Intuit the values of the one-sided…

Observe the following graph representing the RAM memory usage (in GB) of a process server as a function of time $t$ (in seconds). Intuit the values of the one-sided limits at time $t = 4$ 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

0/0 indeterminate forms: factor or use the conjugate

When substituting gives $\frac00$, the limit may still exist. You have to simplify first.

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A technical inspection drone follows an approach path to an antenna whose height is given by the function $H(x) = \frac{x^2 - 9}{x - 3}$, where $x$ is the horizontal…

A technical inspection drone follows an approach path 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 $x = 3$ meters.

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

Solve
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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 asymptote: where the function shoots off

Graph of a function that shoots off toward plus infinity on the left and toward minus infinity on the right of the vertical asymptote x = x₀. x = x₀ both sides escape toward ±∞

To find out which infinity it goes toward, analyze the sign of the quotient on each side of $x_0$.

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

Given the rational function $f(x) = \frac{5}{x - 2}$, which describes the intensity of a magnetic field relative 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
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Given the function $f(x) = \dfrac{x^2 - k}{x^2 - 4x + 4}$: a) Determine whether there is any value of $k$ for which the function has a removable discontinuity at…

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

  1. a) Determine whether there is any value of $k$ for which 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 nearby using one-sided limits.
Solve

Horizontal asymptote: when the limit at infinity is finite

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

Degrees Limit at $\pm\infty$ Horizontal asymptote
$\text{degree}(P)<\text{degree}(Q)$ $0$ $y=0$
$\text{degree}(P)=\text{degree}(Q)$ ratio of leading coefficients $y=\dfrac{a_n}{b_n}$
$\text{degree}(P)>\text{degree}(Q)$ $\pm\infty$ none

Why? For very large $x$, in each polynomial the highest-degree term dominates, so $f(x)$ looks like the quotient of those two terms.

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In a biological study, the number of individuals in a bacterial population (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 bacterial population (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
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Given the function $f(x) = \frac{x^2}{x^2 + 4}$, which models the efficiency of a photovoltaic panel as a function of the radiation received $x$, 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 radiation received $x$, justify by calculating limits whether the function has any horizontal asymptote, and sketch it graphically.

Solve
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A team of engineers is monitoring the temperature $T$ (in degrees Celsius) of a new graphics processor as a function of time $t$ (in minutes) since it was switched on.…

A team of engineers is monitoring the temperature $T$ (in degrees Celsius) of a new graphics processor as a function of time $t$ (in minutes) since it was switched 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 evaluating limits, whether the temperature stabilises in the long term. What type of asymptote does this stabilised value represent?

Solve

Horizontal asymptote with radicals: infinity minus infinity

With radicals, $\sqrt{x^2+4x}-x$ gives the form $\infty-\infty$. It is resolved with the conjugate.

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Consider the function $f(x) = \sqrt{x^2 + 4x} - x$. a) Calculate $\displaystyle \lim_{x \to +\infty} f(x)$. b) Check whether a horizontal asymptote exists as $x$ tends…

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

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

All the asymptotes of a rational function

To describe a rational function, follow this routine:

  1. Domain and zeros of the denominator. Those that do not make the numerator zero are vertical asymptotes.

  2. One-sided limits at each one (the sign decides whether it is $+\infty$ or $-\infty$).

  3. Limit at $\pm\infty$ by comparing degrees: horizontal (or oblique, see the next section).

Reading asymptotes from a graph. If the curve hugs a vertical line $x=x_0$ and goes up or down without bound, that means $\lim_{x\to x_0^\pm}f=\pm\infty$. If at the right or left edge the curve flattens out at height $L$, then $\lim_{x\to\pm\infty}f=L$ and $y=L$ is a horizontal asymptote.

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Find all asymptotes (vertical and horizontal) of the function $f(x) = \frac{3x - 6}{x + 1}$. To do so: 1. Identify the value of $x$ that makes the denominator zero. 2.…

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

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

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

Solve
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Observe the following graph of a function $f(x)$ (each tick mark on the axes corresponds to one unit). Assume that the two branches continue outside the box with the…

Observe the following graph of a function $f(x)$ (each tick mark on the axes corresponds to one unit). Assume that the two branches continue outside the box with the trend they show. From the visual information, determine the value of the following limits and the equations of all its asymptotes:

  1. a) $\displaystyle \lim_{x \to -\infty} f(x)$
  1. b) $\displaystyle \lim_{x \to 1^-} f(x)$ and $\displaystyle \lim_{x \to 1^+} f(x)$
  1. c) $\displaystyle \lim_{x \to +\infty} f(x)$
Graph of a function f(x) made up of two separate branches, drawn on axes with one tick mark per unit −4 −3 −2 −1 1 2 3 4 0 3 2 1 −1 −2 −3 x y f(x)
Solve
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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 on the line $y = 3$. Once the value of $a$ has been 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

Oblique asymptote: when there is exactly one extra degree

Graph of a function approaching its oblique asymptote, staying above it toward the right. y = mx + n

The position of the curve relative to the asymptote is determined by the sign of the remainder $\dfrac{R}{Q(x)}$ at each infinity.

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

Solve
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Consider the function $f(x)$, defined by means of an absolute value, which models the trajectory of a surveillance drone: $$f(x) = \frac{x \times |x-2|}{x+1}$$ Study the…

Consider the function $f(x)$, defined by means of an absolute value, which models the trajectory of a surveillance drone:

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

Study the existence of oblique asymptotes of this function. If they exist, calculate their equations and analyze the position of the curve relative to those asymptotes both at $+\infty$ and at $-\infty$.

Solve
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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 $(0, 2)$ with a gradient of $3$. Once found, check whether the function has any vertical asymptote and justify whether it is removable or not.

Solve

Logarithms and the hierarchy of infinities

Logarithms have a vertical asymptote at the ends of the domain where the argument tends to $0^+$ (or to $+\infty$).

To compare how fast things grow as $x\to+\infty$, the hierarchy of infinities is:

$$\ln x\;\ll\; x^p\;\ll\; e^{x}$$

That is, the exponential beats any power and a power beats the logarithm. Why? Each time $x$ increases, $e^x$ gets multiplied by a fixed number, while $x^p$ only grows a little.

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Find the domain and all 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 point…

Find the domain and all 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 point where the function is undefined, 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

You can do it on paper and send a photo with your answer.

Solve
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In the study of the spread of a computer virus in 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 in 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 have any horizontal asymptote? Justify your answer by applying the corresponding hierarchies of infinities.

Solve

Asymptotes with parameters and piecewise functions

Often a parameter is solved for by requiring a certain asymptote to exist. The idea is the same: cancel the term that shoots off and equate the limit.

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A communications satellite follows an orbit whose gravitational perturbation is described by the function $g(x)$. The satellite must not stray indefinitely from a…

A communications satellite follows an orbit whose gravitational perturbation is described by the function $g(x)$. The satellite must not stray indefinitely from a predicted 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 $y = 2$ as $x \to +\infty$.

Solve
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Let $f(x)$ be the piecewise function defined as: $$f(x) = \begin{cases} \frac{x^2 - k}{x-2} & \text{if } x < 2 \\ \sqrt{x^2 + ax} - x & \text{if } x \geq 2 \end{cases}$$…

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

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

Extension: the $1^\infty$ form and L'Hôpital's rule

When the base tends to $1$ and the exponent to $\infty$, we use the relationship with $e$:

L'Hôpital's rule. If $\lim\dfrac{f(x)}{g(x)}$ gives $\frac00$ or $\frac\infty\infty$ and the derivatives exist, then $\lim\dfrac{f(x)}{g(x)}=\lim\dfrac{f'(x)}{g'(x)}$ (if the latter exists). It can be applied several times.

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Calculate the value of the following limit, which leads to an indeterminate form of type $1^\infty$, using its relation to the number $e$: $$\lim_{x \to +\infty} \left(…

Calculate the value of the following limit, which leads to an indeterminate form of type $1^\infty$, using its relation to the number $e$:

$$\lim_{x \to +\infty} \left( \frac{3x + 5}{3x - 2} \right)^{2x + 1}$$
Solve
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A new data compression algorithm for a streaming platform is being designed. The system response time, in milliseconds, is modelled by the function $T(x)$, where $x$…

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

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

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

Solve
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Calculate the value of the following limit using L'Hôpital's Rule after performing the necessary transformations to resolve the indeterminate form: $$\lim_{x \to 0}…

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

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

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

Solve

Summary

Back to the beginning: the store's response time levels off if its model has a horizontal asymptote; it shoots off at saturation if it has a vertical asymptote; and it grows almost linearly if it has an oblique one.

See all the exercises in this section