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AP Calc AB · Mathematics
Limits and Continuity · Limits

Continuity at a Point

Before you start, make sure you know: Continuity of functions (Precalculus)
Before you start, make sure you know: Modulus function (Precalculus)

A drone climbing from 10 ft to 50 ft passes through every height in between, so its height is a smooth function of time. A parking garage that charges $\$5$ for up to 2 hours and $\$8$ after that never charges $\$6.50$, so its fee function has a break. Calculus needs a precise test for "no break" at a point, one you can write as an AP-style justification. How can limits prove that a graph can be traced through a point without lifting your pencil?

The Three Conditions

Continuity at a point is a short checklist. If one item fails, the function is discontinuous there.

Graph of a continuous function: a smooth curve passes through the filled point (a, f(a)) with no break x = a f(a) limit = f(a): no break

Why three conditions? The graph must have a point at $x=a$ (condition 1). The two sides of the curve must head toward the same height (condition 2). The point must sit exactly at that height (condition 3).

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Check if the function $f(x) = e^{x-1}$ is continuous at the point $x = 1$. For the answer to be complete, you must verify whether the value of the function at the point…

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

Solve
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Given the following piecewise function, calculate the one-sided limits at the breakpoint $x = 3$ and determine whether the function is continuous at that point:…

Given the following piecewise function, calculate the one-sided limits at the breakpoint $x = 3$ 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 a height trajectory defined by the function $h(t)$ (in feet) with respect to time $t$ (in seconds). Due to a change in the control system,…

A surveillance drone follows a height trajectory defined by the function $h(t)$ (in feet) 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 if the height transition is continuous at the instant $t = 5$ to prevent the drone from suffering a sudden movement.

Solve

Reading Discontinuities from a Graph

When a graph is given, ask three questions at $x=a$. Is there a filled dot at $x=a$? Where does the curve head from the left? Where does it head from the right?

Four graphs at x = a: a removable discontinuity with a hole, a jump discontinuity, an infinite discontinuity with a vertical asymptote, and a continuous graph L f(a) Removable: f(a) ≠ L left limit right limit Jump: limits differ x = a Infinite: vertical asymptote Continuous: no break

Remember the notation. $\lim_{x\to a^-}$ uses only values with $xa$. The two-sided limit exists exactly when both one-sided limits exist and are equal.

Some problems call a jump a finite jump and an infinite discontinuity an infinite jump. These are the same ideas.

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Observe the following graph of the function $f(x)$ and analyze its behavior at the point with abscissa $x = 2$. Indicate whether the function is continuous at that point…

Observe the following graph of the function $f(x)$ and analyze its behavior at the point with abscissa $x = 2$. Indicate whether the function is continuous at that point by checking the three conditions for continuity: existence of $f(2)$, existence of the $\lim_{x \to 2} f(x)$ and the coincidence 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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Visually analyze the continuity of the function represented in the following graph. Identify the value of $x$ where the function presents a discontinuity and classify it…

Visually analyze the continuity of the function represented in the following graph. Identify the value of $x$ where the function presents a discontinuity and classify it according to the behavior of its lateral 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 this, 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 this, 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 presents.

Solve

Classifying Discontinuities Algebraically

Graphs are not always given. For a rational function, factor first. Each factor that makes the denominator $0$ is a suspect point. A factor that also cancels with the numerator gives a removable discontinuity. A factor that survives in the denominator gives an infinite one.

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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 presents a discontinuity, indicate what type it is (removable, finite…

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

Solve
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Classify all the discontinuities of the function $g(x)$ and determine its vertical asymptotes by analyzing the behavior of the function in the vicinity of the values…

Classify all the discontinuities of the function $g(x)$ and determine its vertical asymptotes by analyzing the behavior of the function in the vicinity of 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

Absolute Values

The absolute value changes its formula at the point where its inside is $0$, just like a piecewise function. To study continuity there, remove the bars separately on each side and compare one-sided limits.

Graph of y = |x|/x: the horizontal ray y = -1 for x < 0 and the ray y = 1 for x > 0, with open circles at x = 0, showing a jump y = −1 for x < 0 y = 1 for x > 0 x = 0
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Study the continuity of the following function over its entire domain, classifying the points of discontinuity you find: $$f(x) = \frac{|x - 2|}{x^2 - x - 2}$$ Accompany…

Study the continuity of the following function over its entire domain, classifying the points of discontinuity you find:

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

Accompany your analytical analysis with a sketch of the one-sided limits at the conflicting 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, indicate 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, indicate what type they are and if there is any way to redefine the function to eliminate them.

Solve
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Study the continuity of the following function over its entire domain, classifying each of its points of discontinuity in detail: $f(x) = \frac{|x^2 - 1|}{x^2 - 3x + 2}$…

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

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

Subsequently, propose a redefinition for the function at those points where the discontinuity is removable to transform it into a continuous function at those points.

x f(x) 0 1 2 1 2 −2 (1, 2) (1, −2) x = 2 y = 1
Solve

Exponentials: One-Sided Limits That Differ

Some functions have a jump even though nothing "blows up". A typical case is an exponential whose exponent is $\dfrac1x$. As $x\to0^-$ the exponent goes to $-\infty$, and as $x\to0^+$ it goes to $+\infty$.

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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 this, 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 this, 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 present at this point?

Solve

Piecewise Functions: Finding Parameters

Problems often include unknown constants and ask for the values that make a piecewise function continuous. Think of welding the pieces together: at every breakpoint they must meet at the same height.

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

Determine the value of the parameter $k$ so that the function $g(x)$ is continuous over 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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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 \cdot 2^x & \text{if } x \geq…

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

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

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

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

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

Explain in detail the process followed, verifying the equality of the one-sided limits at each branch change point.

Solve

Parameters and Removable Discontinuities

Sometimes a parameter controls whether a discontinuity is removable or infinite. The key fact is below.

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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 presents a removable discontinuity at $x = 3$. Is there any value of $k$ that makes it continuous at that point? Reason your answer by calculating one-sided limits.

Solve
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Given the function dependent 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 dependent 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 if there is any vertical asymptote.
  3. c) Is there any value of $k$ that makes the function continuous over all $\mathbb{R}$? Justify your answer.
Solve

Limit Tools: Conjugates, Special Limits, and $1^\infty$

Harder continuity problems need limits that give $\frac00$ or $1^\infty$ when you substitute. Here are three tools.

Multiply by the conjugate

When a square root appears in a $\frac00$ limit, multiply the top and bottom by the conjugate. Use $(\sqrt{A}-B)(\sqrt{A}+B)=A-B^2$.

Two special limits

For example, $\displaystyle\lim_{x\to0}\frac{e^{3x}-1}{x}=3$. Quotients of continuous functions with a nonzero denominator need no tool. For instance, $\displaystyle\lim_{T\to0}\frac{e^T}{e^T+1}=\frac{1}{2}$ by direct substitution.

The form $1^\infty$

For example, $\displaystyle\lim_{x\to\infty}\left(\frac{x+2}{x}\right)^{3x}=\lim\left(1+\frac2x\right)^{3x}$. Here $f\cdot g=\frac2x\cdot3x=6$, so the limit is $e^6$.

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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 suffer sudden jumps (is continuous) at the instant $t = 2$ s.

1 2 3 4 5 6 2 4 6 8 10 t (s) a (ft/s²) a(t) = t² + m a(t) = (t - 2) / (√(t + 7) - 3) Limit = 6
Solve
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An industrial cooling system regulates its power $P$ (in kilowatts) according to the outside temperature $T$ (in degrees Fahrenheit) using the following function: $$P(T)…

An industrial cooling system regulates its power $P$ (in kilowatts) according to the outside temperature $T$ (in degrees Fahrenheit) using the following function:

$$P(T) = \begin{cases} \frac{a \cdot e^T}{e^T + 1} & \text{if } T \leq 0 \\ \frac{\sqrt{T+4} - 2}{T} & \text{if } T > 0 \end{cases}$$

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

−4 −2 2 4 6 8 10 0.1 0.2 0.3 0.4 T [ºF] P [kW] a = 1 (Discontinuous) T ≤ 0 T > 0 P(0) = 0.25 a = 0.5
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 $a \in \mathbb{R}$ so 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 $a \in \mathbb{R}$ so 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:

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

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

Solve

Putting It Together: Harder Parameter Problems

Combine the two ideas: pick the right tool for each one-sided limit, then apply the three conditions at the breakpoint. The value $f(a)$ is often given by its own formula, such as $b$ or $\frac{a}{12}$.

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

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

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

Determine the values of the real parameters $a$ and $b$ so that the intensity of the magnetic field 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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A telecommunications company is adjusting the power of a 5G antenna. The signal loss $L$ (in decibels) depends on the distance $d$ (in miles) according to the…

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

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

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

Solve
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A team of robotics engineers has programmed the movement of a mechanical arm. The velocity of the end of the arm (in ft/s) follows the trajectory:…

A team of robotics engineers has programmed the movement of a mechanical arm. The velocity of the end of the arm (in ft/s) follows the trajectory:

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

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

Solve

The Intermediate Value Theorem

Continuity has a powerful consequence. A continuous function cannot skip values.

A continuous curve goes from a point below the x-axis at x = a to a point above the x-axis at x = b, so it must cross the axis at some c in between a b c f(a) < 0 f(b) > 0 f(c) = 0

Why does it work? To go from below the axis to above it without lifting the pencil, the curve has to cross the axis. A break (discontinuity) is the only way to skip over it.

To show that two graphs $f$ and $g$ intersect, apply the theorem to $h(x)=f(x)-g(x)$. A zero of $h$ is a point where $f(c)=g(c)$.

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

Apply Bolzano's Theorem to prove that the graphs of the functions $f(x) = \ln(x)$ and $g(x) = e^{-x}$ intersect at at least one point 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 start: the drone's height is continuous, so it passes through every height between 10 ft and 50 ft. The parking fee has a jump at the 2-hour mark ($\lim_{t\to2^-}=5$ but $\lim_{t\to2^+}=8$), so it never takes the value $\$6.50$.

Summary

See all the exercises in this section