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Limits and continuity · Limits

Asymptotes of a function

An asymptote is a line that the graph gets as close to as you like, almost never actually touching it. To fully describe a rational, logarithmic or irrational function in the EBAU you need to find its three possible asymptotes and, in addition, know on which side the curve approaches each one.

Before you start, make sure you know: Calculating limits of functions (1st Bachillerato)
Before you start, make sure you know: Limit of a function as x → ∞ (1st Bachillerato)

Vertical asymptote: where the function shoots off

Graph of a function that diverges to plus and minus infinity on both sides of the vertical asymptote x = x₀. x = x₀ both sides "escape" towards ±∞
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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 centre. Calculate the limit of the height when the drone is very close to $x = 3$ metres.

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

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 behaviour 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

Horizontal asymptote: when the limit at infinity is finite

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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
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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

Oblique asymptote: when there is exactly one degree to spare

Graph of a function approaching its oblique asymptote from above at plus infinity and from below at minus infinity. y = mx + n
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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: $$f(x) = \frac{x \cdot |x-2|}{x+1}$$ Study the…

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

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

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

Asymptotes with logarithms and with parameters

Logarithmic functions also have vertical asymptotes at the finite ends of their domain where the argument tends to $0^+$ ($\ln\to-\infty$) or to $+\infty$, for example when the denominator of the argument vanishes ($\ln\to+\infty$). And in the EBAU it is common for an unknown parameter to be solved from the required asymptote.

Graph of a function with logarithms showing branches that diverge towards vertical asymptotes at the ends of the intervals of the domain. x = −2 x = −1 x = 1 x = 2 the logarithm shoots off to ±∞ at the edges of the domain
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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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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 behaviour 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.

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The full picture: the three asymptotes of a function

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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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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

Summary

See all the exercises in this section