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Secondary 6 · Mathematics
Limits and continuity · Limits

Continuity of a Function at a Point

Before you start, make sure you know: Absolute value function (Secondary Year 5 (second cycle, academic track))

Imagine that Wilkin is flying a drone over the Malecón and the computer shows him its height as a function of time. If the graph has a jump, the drone "teleports" from one height to another, which is impossible. How do you check with calculations that a function has no jumps or holes at a point? That is what this section is about.

The three conditions for continuity

Put simply: you can draw the graph near $a$ without lifting your pencil.

Continuous graph at x = a: a smooth curve passing through the filled point (a, f(a)) without breaks x = a f(a) limit = f(a): no break

Polynomials, exponentials, sine, and cosine are continuous on all of $\mathbb{R}$; roots and logarithms are continuous on their domain; rational functions are continuous wherever the denominator is not zero. That is why, for these functions, the limit is found by substituting.

When a condition fails

Depending on which condition fails, the discontinuity is classified like this:

Type What happens Example
Removable The limit exists and is finite, but $f(a)$ does not exist or is different from the limit $\dfrac{x^2-1}{x-1}$ at $x=1$
Finite jump The one-sided limits exist, are finite, and are different $\dfrac{\lvert x\rvert}{x}$ at $x=0$
Infinite jump At least one one-sided limit is $+\infty$ or $-\infty$ (vertical asymptote) $\dfrac{1}{x+4}$ at $x=-4$
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Observe the following graph of the function $f(x)$ and analyse its behavior at the point with x-coordinate $x = 2$. State whether the function is continuous at that…

Observe the following graph of the function $f(x)$ and analyse its behavior at the point with x-coordinate $x = 2$. State whether the function is continuous at that point by checking the three conditions for continuity: existence of $f(2)$, existence of $\lim_{x \to 2} f(x)$ and the equality of both values.

x→2⁻ x→2⁺ f(x)→2 f(x)→3 1 2 3 4 5 1 2 3 4 x y f(x)
Solve
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Check whether the function $f(x) = e^{x-1}$ is continuous at the point $x = 1$. For a complete answer, you must verify whether the value of the function at that point…

Check whether the function $f(x) = e^{x-1}$ is continuous at the point $x = 1$. For a complete answer, you must verify whether the value of the function at that point coincides with the limit of the function as $x$ approaches 1.

Solve
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Study the continuity of the function $f(x) = \frac{x^2 - 1}{x - 1}$ at the point $x = 1$. If it has a discontinuity, state what type it is (removable, jump, or infinite)…

Study the continuity of the function $f(x) = \frac{x^2 - 1}{x - 1}$ at the point $x = 1$. If it has a discontinuity, state what type it is (removable, jump, or infinite) and explain whether there is any way to redefine the function so that it is continuous at that point.

Solve

Infinite jump and graphs

When the denominator becomes zero and the numerator does not, the function grows or decreases without bound near that point. You have to look at the sign on each side.

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Visually analyse the continuity of the function represented in the following graph. Identify the value of $x$ where the function has a discontinuity and classify it…

Visually analyse the continuity of the function represented in the following graph. Identify the value of $x$ where the function has a discontinuity and classify it based on the behavior of its left-hand and right-hand branches.

x = −2 f(x) x y −4 −3 −1 1 2 3 4 1 −1 0
Solve
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Given the function $f(x) = \frac{1}{x + 4}$, justify why it is not continuous at the point $x = -4$. To do so, calculate the one-sided limits $\lim_{x \to -4^-} f(x)$…

Given the function $f(x) = \frac{1}{x + 4}$, justify why it is not continuous at the point $x = -4$. To do so, calculate the one-sided limits $\lim_{x \to -4^-} f(x)$ and $\lim_{x \to -4^+} f(x)$ and describe the type of discontinuity it has.

Solve

Piecewise functions and absolute value

At a point where the function changes formula, you have to calculate the two one-sided limits using the corresponding formula for each side. The absolute value also changes expression where its argument becomes zero.

Finite jump at x = 2: the left branch rises to −1/3, below the axis, and the right branch starts at 1/3, above the axis, and goes down toward 0 x = 2 left: tends to −1/3 right: starts at 1/3 the one-sided limits differ: finite jump
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Given the following piecewise function, calculate the left-hand and right-hand limits at the boundary point $x = 3$ and determine whether the function is continuous at…

Given the following piecewise function, calculate the left-hand and right-hand limits at the boundary point $x = 3$ and determine whether the function is continuous at that point:

$f(x) = \begin{cases} 2x - 4 & \text{if } x < 3 \\ \sqrt{x + 1} & \text{if } x \ge 3 \end{cases}$

Solve
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A surveillance drone follows an altitude trajectory defined by the function $h(t)$ (in meters) with respect to time $t$ (in seconds). Due to a change in the control…

A surveillance drone follows an altitude trajectory defined by the function $h(t)$ (in meters) with respect to time $t$ (in seconds). Due to a change in the control system, the function changes at $t = 5$:

$h(t) = \begin{cases} t^2 - 10 & \text{if } 0 \le t < 5 \\ 3t & \text{if } t \ge 5 \end{cases}$

Check whether the transition in altitude is continuous at time $t = 5$ to prevent the drone from undergoing a sudden movement.

Solve
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Study the continuity of the following function across its domain, classifying any points of discontinuity you find: $$f(x) = \frac{|x - 2|}{x^2 - x - 2}$$ Accompany your…

Study the continuity of the following function across its domain, classifying any points of discontinuity you find:

$$f(x) = \frac{|x - 2|}{x^2 - x - 2}$$

Accompany your analytical study with a diagram of the one-sided limits at the critical points.

Solve
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Given the function: $$f(x) = \frac{x^2 - 1}{|x| - 1}$$ Study its continuity at the points $x = 1$ and $x = -1$. If there are discontinuities, state what type they are…

Given the function:

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

Study its continuity at the points $x = 1$ and $x = -1$. If there are discontinuities, state what type they are and whether there is any way to redefine the function to remove them.

Solve

Exponentials, logarithms, and roots

With exponentials or logarithms, the one-sided limits can give different results even if the function looks harmless.

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Study the continuity of the function $f(x) = \frac{1}{1 + 2^{1/x}}$ at the point $x = 0$. To do so, calculate the one-sided limits $\lim_{x \to 0^-} f(x)$ and…

Study the continuity of the function $f(x) = \frac{1}{1 + 2^{1/x}}$ at the point $x = 0$. To do so, calculate the one-sided limits $\lim_{x \to 0^-} f(x)$ and $\lim_{x \to 0^+} f(x)$. What type of discontinuity does the function have at that point?

Solve
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Study the continuity of the following function over its entire domain, classifying in detail each of its points of discontinuity:…

Study the continuity of the following function over its entire domain, classifying in detail each of its points of discontinuity:

$f(x) = \dfrac{|x^2 - 1|}{x^2 - 3x + 2}$

Afterwards, if there is any point where the discontinuity is removable, propose a redefinition of the function at that point to turn it into a function that is continuous there. If there is none, justify it.

Solve
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Classify all the discontinuities of the function $g(x)$ and determine its vertical asymptotes by analysing the behavior of the function near the values where it is not…

Classify all the discontinuities of the function $g(x)$ and determine its vertical asymptotes by analysing the behavior of the function near the values where it is not defined:

$$g(x) = \frac{x^3 - 4x}{x^2 - 2x}$$

How would you define the function at the points of removable discontinuity so that it becomes continuous at them?

Solve

Finding parameters that give continuity

A very common problem: a piecewise function with unknown letters, and you must find the values that remove every jump at the junctions.

If the limit of one piece is an indeterminate form $0/0$, first resolve it (by factoring or multiplying by the conjugate), and then set the expressions equal.

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Determine the value of the parameter $k$ so that the function $g(x)$ is continuous on the entire real line $\mathbb{R}$:…

Determine the value of the parameter $k$ so that the function $g(x)$ is continuous on the entire real line $\mathbb{R}$:

$g(x) = \begin{cases} x + k & \text{if } x \le 2 \\ x^2 - 1 & \text{if } x > 2 \end{cases}$

Solve
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Given the function dependent on the parameter $k \in \mathbb{R}$: $$f(x) = \begin{cases} \frac{x^2 - kx}{x - 3} & \text{if } x < 3 \\ 2x - 5 & \text{if } x \geq 3…

Given the function dependent on the parameter $k \in \mathbb{R}$:

$$f(x) = \begin{cases} \frac{x^2 - kx}{x - 3} & \text{if } x < 3 \\ 2x - 5 & \text{if } x \geq 3 \end{cases}$$

Calculate the value of $k$ so that the function has a jump discontinuity (finite jump) at $x = 3$. Is there any value of $k$ that makes it continuous at that point? Justify your answer by computing the one-sided limits.

Solve
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An industrial cooling system regulates its power $P$ (in kilowatts) according to the outside temperature $T$ (in degrees Celsius) by means of the following function:…

An industrial cooling system regulates its power $P$ (in kilowatts) according to the outside temperature $T$ (in degrees Celsius) by means of the following function:

$$P(T) = \begin{cases} \dfrac{a \times e^T}{e^T + 1} & \text{if } T \leq 0 \\[2mm] \dfrac{\sqrt{T+4} - 2}{T} & \text{if } T > 0 \end{cases}$$

Determine the value of the constant $a$ so that the power transition is continuous when the temperature passes through $0\,^\circ C$.

Solve
Practice★★☆Not started
Consider the function $f(x)$ defined by: $$f(x) = \begin{cases} \ln(x+1) & \text{if } -1 < x < 0 \\ x^2 - a & \text{if } 0 \leq x < 2 \\ b \times 2^x & \text{if } x \geq…

Consider the function $f(x)$ defined by:

$$f(x) = \begin{cases} \ln(x+1) & \text{if } -1 < x < 0 \\ x^2 - a & \text{if } 0 \leq x < 2 \\ b \times 2^x & \text{if } x \geq 2 \end{cases}$$

Find the values of $a$ and $b$ so that the function is continuous on its domain. Once found, state whether the function is continuous at $x = -1$.

Solve
Practice★★☆Not started
A new engine prototype is being tested whose acceleration $a$ (in $m/s^2$) depends on time $t$ (in seconds) according to the expression: $$a(t) = \begin{cases} t^2 + m &…

A new engine prototype is being tested whose acceleration $a$ (in $m/s^2$) depends on time $t$ (in seconds) according to the expression:

$$a(t) = \begin{cases} t^2 + m & \text{if } 0 \leq t < 2 \\ \frac{t-2}{\sqrt{t+7} - 3} & \text{if } t > 2 \end{cases}$$

Calculate the value of $m$ so that the acceleration does not undergo sudden jumps (is continuous) at time $t = 2$ s.

1 2 3 4 5 6 2 4 6 8 10 t (s) a (m/s²) a(t) = t² + m a(t) = (t - 2) / (√(t + 7) - 3) Limit = 6
Solve
Practice★★☆Not started
Determine the values of $a$ and $b$ so that the function $f(x)$ is continuous everywhere in $\mathbb{R}$: $$f(x) = \begin{cases} \frac{x+2}{x-1} & \text{if } x < -2 \\ a…

Determine the values of $a$ and $b$ so that the function $f(x)$ is continuous everywhere in $\mathbb{R}$:

$$f(x) = \begin{cases} \frac{x+2}{x-1} & \text{if } x < -2 \\ a \times x + b & \text{if } -2 \leq x \leq 3 \\ \sqrt{x^2 + 7} & \text{if } x > 3 \end{cases}$$

Explain in detail the process followed, checking that the left-hand and right-hand limits are equal at each boundary point.

Solve
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A team of robotics engineers has programmed the motion of a mechanical arm. The speed of the end of the arm (in m/s) follows the path:…

A team of robotics engineers has programmed the motion of a mechanical arm. The speed of the end of the arm (in m/s) follows the path:

$v(t) = \begin{cases} \frac{\sqrt{t+5}-3}{t-4} & \text{if } 0 \leq t < 4 \\ \frac{a}{12} & \text{if } t = 4 \\ \frac{t^2 - 16}{t^2 - 3t - 4} \times b & \text{if } t > 4 \end{cases}$

Determine the values of $a$ and $b$ so that the robot's motion is continuous (without sudden jerks) at time $t = 4$ seconds.

Solve

Limits involving $e$ that appear in the parameters

Some parameter problems hide a limit with logarithms, exponentials, or powers. They are solved with these results.

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A telecommunications company is adjusting the power of a 5G antenna. The signal loss $L$ (in decibels) depends on the distance $d$ (in km) according to the expression:…

A telecommunications company is adjusting the power of a 5G antenna. The signal loss $L$ (in decibels) depends on the distance $d$ (in km) according to the expression:

$L(d) = \begin{cases} \frac{k \times \ln(d+1)}{d} & \text{if } 0 < d < 1 \\ \sqrt{d^2 + 3} - a & \text{if } d \geq 1 \end{cases}$

The power transition is required to be smooth and continuous so as not to interrupt calls at the 1 km boundary. Calculate the values of $k$ and $a$ knowing that as the distance tends to 0 km, the signal loss tends to 2 dB.

Solve
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In the design of a new component for a particle accelerator, the magnetic field strength as a function of position $x$ (in millimeters) is defined by the following…

In the design of a new component for a particle accelerator, the magnetic field strength as a function of position $x$ (in millimeters) is defined by the following function:

$f(x) = \begin{cases} \frac{e^{ax} - 1}{x} & \text{if } x < 0 \\ b & \text{if } x = 0 \\ \frac{x^2 - 4}{x - 2} \times \cos(\pi x) & \text{if } x > 0 \end{cases}$

Determine the values of the real parameters $a$ and $b$ so that the magnetic field strength is continuous at the point $x = 0$. Once found, would the function be continuous at the point $x = 2$? Reason your answer analytically.

Solve
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Given the function $f(x) = \left( \frac{2x^2 + 3}{2x^2 - 1} \right)^{ax^2}$: a) Find the value of the parameter $a \in \mathbb{R}$ such that…

Given the function $f(x) = \left( \frac{2x^2 + 3}{2x^2 - 1} \right)^{ax^2}$:

  1. a) Find the value of the parameter $a \in \mathbb{R}$ such that $\lim_{x \to \infty} f(x) = e^2$.
  2. b) With the value of $a$ obtained in the previous section, define a new function $g(x)$ such that:

$g(x) = \begin{cases} f(x) & \text{if } x \ge 1 \\ \frac{x^2 + bx + c}{x - 1} & \text{if } x < 1 \end{cases}$

Determine $b$ and $c$ so that $g(x)$ is continuous at $x = 1$.

Solve
Practice★★★Not started
Given the function depending on the parameter $k$: $f(x) = \frac{x^3 - kx^2 + 3x}{x^2 - x}$ a) Find the value of $k$ so that the function has a removable discontinuity…

Given the function depending on the parameter $k$:

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

  1. a) Find the value of $k$ so that the function has a removable discontinuity at $x = 1$.
  2. b) For that value of $k$, study the continuity at $x = 0$ and determine whether any vertical asymptote exists.
  3. c) Does any value of $k$ exist that makes the function continuous across all of $\mathbb{R}$? Justify your answer.
Solve

Extension: the Bolzano theorem

If you walk from the Malecón to the Monumento de Santiago, starting below sea level and arriving above it, with no jumps you must have passed through sea level. That is what Bolzano says.

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Apply Bolzano's Theorem to show that the graphs of the functions $f(x) = \ln(x)$ and $g(x) = e^{-x}$ intersect at least once within the interval $(1, e)$. Justify with…

Apply Bolzano's Theorem to show that the graphs of the functions $f(x) = \ln(x)$ and $g(x) = e^{-x}$ intersect at least once within the interval $(1, e)$. Justify with reasoning whether this point of intersection is unique in that interval based on the growth of the functions.

x y 0 1 e 1 c f(x) = ln(x) g(x) = e⁻ˣ f(1) < g(1) f(e) > g(e)
Solve

Back to the beginning: Wilkin's drone does not "teleport" when, at a junction, the one-sided limits and the value of the height coincide; that is exactly continuity.

Summary

See all the exercises in this section