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Precalculus · Mathematics
Functions · Types of limits and continuity

Continuity of a Function

Before you start, make sure you know: Piecewise functions (Algebra 1)
Before you start, make sure you know: Algebraic fractions (Algebra 1)

Hike up a hill and your elevation changes with no gaps: at every instant there is an elevation, and a moment later it is almost the same. A downtown parking garage is different. At 59 minutes you pay one price, and a minute later the price jumps. The first situation is continuous; the second is not. In this section you turn that "no lifting your pencil" idea into a precise test using limits.

Continuity at a Point

A function is continuous at $x=a$ if its graph does not break there. The value the function approaches from both sides must be the value it actually takes at $a$.

Graph of a function that is continuous at x = a: the curve passes through the point (a, f(a)) with no gap or jump. x = a f(a) no break: the limit equals f(a)

When a function changes formula at $x=a$, the two sides come from different formulas, so you check them separately. A function is continuous from the left at $a$ if $\lim_{x\to a^-} f(x)=f(a)$, and continuous from the right if $\lim_{x\to a^+} f(x)=f(a)$. Being continuous at $a$ means both are true.

Practice★☆☆Not started
Given the piecewise function: $f(x) = \begin{cases} 2x^2 - 3 & \text{if } x < 2 \\ 5x - 5 & \text{if } x \ge 2 \end{cases}$ Determine if $f(x)$ is continuous at the…

Given the piecewise function:

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

Determine if $f(x)$ is continuous at the point $x = 2$.

Solve

Types of Discontinuity

When at least one of the three conditions fails, $f$ is discontinuous at that point. Three types come up constantly. They are not the only possible ones (for example $\sin(1/x)$ oscillates forever as $x\to0$), but they are the ones you need here.

Three graphs side by side showing a removable discontinuity with a hole, a jump discontinuity with two branches at different heights, and an infinite discontinuity with a vertical asymptote. removable jump infinite

On a graph, an open circle marks a point that is missing or not part of that piece, and a filled circle marks the actual value. A hole means removable; two pieces at different heights mean a jump; a curve shooting off along a vertical line means infinite.

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Observe the graph of the function $f(x)$. Identify the points where the function presents discontinuities and classify the type of discontinuity at each of them.

Observe the graph of the function $f(x)$. Identify the points where the function presents discontinuities and classify the type of discontinuity at each of them.

X Y 0 1 4 −2 2 4 f(x)
Solve
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The rational function $f(x) = \frac{x^2 + 2x - 8}{x - 2}$ is not defined at $x = 2$. What numerical value should $f(2)$ take for the function to be continuous at this…

The rational function $f(x) = \frac{x^2 + 2x - 8}{x - 2}$ is not defined at $x = 2$. What numerical value should $f(2)$ take for the function to be continuous at this point?

Solve

Where Familiar Functions Are Continuous

You will not check three conditions every time. Most functions you know are continuous everywhere they are defined.

Function Continuous on
Polynomial all real numbers
Rational $\dfrac{p(x)}{q(x)}$ every $x$ with $q(x)\neq 0$
$\sqrt{g(x)}$ where $g(x)\ge 0$
$e^{g(x)}$, $\sin$, $\cos$, $\lvert x\rvert$ all real numbers (when $g$ is continuous)
$\ln g(x)$, $\log g(x)$ where $g(x)>0$

Sums, differences, products, and quotients (with nonzero denominator) of continuous functions are continuous. So is a composition $f(g(x))$ wherever $g$ is continuous at $x$ and $f$ is continuous at $g(x)$.

At the endpoint of a domain like $[4,\infty)$ there is only one side, so "continuous" there means continuous from that side.

Number line showing the domain of the square root of (x - 4)(x + 3): the rays x at most -3 and x at least 4, with closed circles at the endpoints. −3 4 radicand < 0 here
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Determine the domain of continuity of the function with radical $f(x) = \sqrt{x^2 - 5x + 6}$.

Determine the domain of continuity of the function with radical $f(x) = \sqrt{x^2 - 5x + 6}$.

Solve
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Study the continuity of the logarithmic function $f(x) = \ln(8 - 2x)$, clearly indicating the interval of real numbers where it is continuous.

Study the continuity of the logarithmic function $f(x) = \ln(8 - 2x)$, clearly indicating the interval of real numbers where it is continuous.

Solve
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Given the functions $g(x) = \frac{1}{x - 2}$ and $h(x) = \sqrt{x + 1}$, find the analytical expression of the composite function $f(x) = g(h(x))$ and exhaustively study…

Given the functions $g(x) = \frac{1}{x - 2}$ and $h(x) = \sqrt{x + 1}$, find the analytical expression of the composite function $f(x) = g(h(x))$ and exhaustively study its continuity, indicating its domain and the type of discontinuities it presents.

Solve

Rational Functions: Holes and Asymptotes

A rational function can only be discontinuous where the denominator is $0$. At each such $x$, you decide whether you have a hole or an asymptote.

Graph of f(x) = (x − 3)(x + 1) / ((x − 1)(x + 1)) with a vertical asymptote at x = 1 and a hole at the point (−1, 2) where the common factor x + 1 was cancelled. hole at (−1, 2) x = 1
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Study the continuity of the function $f(x) = \frac{x^2-x-6}{x^2-9}$ and classify its points of discontinuity.

Study the continuity of the function $f(x) = \frac{x^2-x-6}{x^2-9}$ and classify its points of discontinuity.

Solve
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Study in detail the continuity of the rational function $f(x) = \frac{x^3 - 3x^2 - x + 3}{x^2 - x - 6}$. Classify the types of discontinuity it presents at the points…

Study in detail the continuity of the rational function $f(x) = \frac{x^3 - 3x^2 - x + 3}{x^2 - x - 6}$. Classify the types of discontinuity it presents at the points where it is not defined.

Solve

Piecewise Functions: Finding One Parameter

If every piece is continuous on its own interval, the only places left to check are the points where the formula changes (the "seams"). Of course, each piece may also have its own trouble spots, such as a denominator that vanishes inside its interval.

To find an unknown parameter, set up the equation "left = right":

  1. Find $f(a)$ from the piece that includes $a$ (the one with $\le$ or $\ge$).

  2. Find the two one-sided limits. Simplify first if you get $\frac00$.

  3. Set them equal and solve for the parameter.

Graph of a piecewise function where a parabola piece and a line piece meet at the same point at x = 2. x = 2 x² + 1 ax − 3 they meet at x = 2, so 2a − 3 = 5
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Calculate the value of $k$ so that the following function is continuous at $x = 1$:…

Calculate the value of $k$ so that the following function is continuous at $x = 1$:

$f(x) = \begin{cases} 3x + k & \text{if } x \le 1 \\ x^2 - 4x + 5 & \text{if } x > 1 \end{cases}$

Solve
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Study the continuity of the following function, calculating the value of $k$ that makes it continuous at the point $x=0$:…

Study the continuity of the following function, calculating the value of $k$ that makes it continuous at the point $x=0$:

$f(x) = \begin{cases} e^{2x} + k & \text{if } x \le 0 \\ \frac{x^2 + 3x}{x} & \text{if } x > 0 \end{cases}$

Solve

Two or More Parameters

With two unknowns you need two equations. Each seam gives one, and so does any extra data point in the problem. Then you solve a system (substitution or elimination).

When a model gives extra information, such as "$c_p(10)=3.5$", plug it into the correct piece to get one more equation. For logarithms remember $\log_{10}100=2$ and $\log_{10}1000=3$.

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Determine the values of the parameters $a$ and $b$ so that the function $f(x)$ is continuous over the entire set of real numbers $\mathbb{R}$:…

Determine the values of the parameters $a$ and $b$ so that the function $f(x)$ is continuous over the entire set of real numbers $\mathbb{R}$:

$f(x) = \begin{cases} x + a & \text{if } x \le -1 \\ x^2 - b & \text{if } -1 < x < 2 \\ 3x - 1 & \text{if } x \ge 2 \end{cases}$

Solve
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Calculate the parameters $a$ and $b$ so that the following function is continuous over its entire domain:…

Calculate the parameters $a$ and $b$ so that the following function is continuous over its entire domain:

$f(x) = \begin{cases} \frac{a}{x - 1} & \text{if } x < 0 \\ x^2 + b & \text{if } 0 \le x \le 2 \\ e^{x - 2} + 1 & \text{if } x > 2 \end{cases}$

Solve
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The specific heat capacity of a gas as a function of temperature $T$ (in Kelvin) is modeled by the following piecewise function:…

The specific heat capacity of a gas as a function of temperature $T$ (in Kelvin) is modeled by the following piecewise function:

$c_p(T) = \begin{cases} a + bT & \text{if } 0 < T \le 100 \\ c \log_{10}(T) & \text{if } T > 100 \end{cases}$

Knowing that $c_p(10) = 3.5$, that the physical model requires the function to be continuous at $T = 100$, and that at a temperature of $1000$ Kelvin the capacity is $12$, determine the values of the parameters $a, b$ and $c$.

Solve

Rational Functions with Parameters

Now the unknown sits inside a rational function, and the type of discontinuity is what you are told. Translate each condition into an algebra statement.

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Given the function $f(x) = \frac{x^2 + kx - 6}{x - 2}$, calculate the value of the parameter $k$ so that the discontinuity at $x = 2$ is removable. Once found, what will…

Given the function $f(x) = \frac{x^2 + kx - 6}{x - 2}$, calculate the value of the parameter $k$ so that the discontinuity at $x = 2$ is removable. Once found, what will be the value of the limit of the function at that point?

Solve
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Find the values of the parameters $a, b, c$ and $d$ so that the rational function $f(x) = \frac{x^2 + ax + b}{x^2 - cx + d}$ simultaneously satisfies the following three…

Find the values of the parameters $a, b, c$ and $d$ so that the rational function $f(x) = \frac{x^2 + ax + b}{x^2 - cx + d}$ simultaneously satisfies the following three conditions:

  • It has a removable discontinuity at $x = 1$.

  • It has an infinite jump discontinuity at $x = -2$.

  • Its graph passes through the origin.

Solve
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Discuss the continuity of the function $f(x) = \frac{x^2 - 1}{x^2 - mx + 4}$ depending on the possible values of the real parameter $m$. Pay special attention to the…

Discuss the continuity of the function $f(x) = \frac{x^2 - 1}{x^2 - mx + 4}$ depending on the possible values of the real parameter $m$. Pay special attention to the cases where the denominator has roots that coincide with those of the numerator.

Solve

Absolute Value and Step Functions

Absolute value

To study $|g(x)|$, rewrite it as a piecewise function by finding where $g(x)$ is positive or negative:

$$|g(x)|=\begin{cases} g(x) & g(x)\ge 0 \\ -g(x) & g(x)<0\end{cases}$$

If $g$ is continuous, $|g|$ is a composition of continuous functions, so $|g|$ is continuous too. A division by an expression that vanishes can still cause trouble, though.

Step functions

Many prices are step functions: the price is constant, then jumps. They show up with the floor function $\lfloor t\rfloor$ (the greatest integer not above $t$).

Parking and shipping prices usually say "or fraction thereof": any part of an hour counts as a whole hour. That is rounding up, and the function that does it is the ceiling function $\lceil t\rceil$, the smallest integer that is not below $t$. For example $\lceil 0.2\rceil=1$, $\lceil 1\rceil=1$, and $\lceil 1.01\rceil=2$. The number of hours you are billed for is $\lceil t\rceil$.

With "or fraction thereof," the boundary belongs to the lower price: exactly 1 hour costs the first-hour price, and 1 hour and 1 second costs more. So on the graph each step has an open dot at the left end and a filled dot at the right end.

Step graph of a parking cost: 3 dollars for up to 1 hour, 5 dollars for up to 2 hours, 7 dollars for up to 3 hours, with an open dot at the left end and a filled dot at the right end of each step. 1 2 3 hr $3 $5 $7

Because each one-sided limit is finite and they differ, every step edge is a jump discontinuity, but between the edges the function is constant, hence continuous.

Back to the start: a garage that charges $\$2.50$ for the first hour has you paying $\$2.50$ at 59 minutes, and the moment you pass 60 minutes the price jumps to the next level. That is exactly a jump discontinuity on a step function.

Practice★☆☆Not started
Express the absolute value function $f(x) = |x^2 - 4x|$ as a piecewise function and study its continuity at the points $x = 0$ and $x = 4$.

Express the absolute value function $f(x) = |x^2 - 4x|$ as a piecewise function and study its continuity at the points $x = 0$ and $x = 4$.

Solve
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A central parking garage charges $\$2.50$ for the first hour or fraction thereof, and $\$1.50$ for each additional hour or fraction thereof. Write the analytical…

A central parking garage charges $\$2.50$ for the first hour or fraction thereof, and $\$1.50$ for each additional hour or fraction thereof.

Write the analytical expression of the step function $C(t)$ representing the cost for a parking time of $0 < t \le 4$ hours. Study its continuity and state what type of discontinuities it presents.

Solve
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Study the continuity of the function $f(x) = \frac{|x^2 - 4|}{x - 2}$. First write the function as a piecewise function without using the absolute value and classify in…

Study the continuity of the function $f(x) = \frac{|x^2 - 4|}{x - 2}$. First write the function as a piecewise function without using the absolute value and classify in detail the discontinuities you find.

Solve

Continuity in Context and from Graphs

Real models are piecewise functions. Continuity says there is no sudden change at the switch. A jump says something happens instantly: a fee is charged, or a dose is injected.

When the data come from a graph, read the key points and turn each into an equation. For a rational graph, translate the features:

  • A vertical asymptote at $x=q$ means $(x-q)$ stays in the denominator.

  • A hole at $x=p$ means $(x-p)$ appears in both numerator and denominator, and the hole's height is the limit.

  • A horizontal asymptote at $y=1$ with equal degrees means the leading coefficients are equal.

  • An $x$-intercept is a zero of the numerator that is not cancelled.

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A cell phone company offers a data plan with the following conditions: a fixed fee of $\$10$ is paid which includes the first $5$ GB. For each additional GB up to $20…

A cell phone company offers a data plan with the following conditions: a fixed fee of $\$10$ is paid which includes the first $5$ GB. For each additional GB up to $20$ GB, $\$2$ is paid. From $20$ GB onwards, the price follows the expression $a + 1.5x$, where $x$ is the total GB consumed.

Find the value of the parameter $a$ so that the function representing the total cost is continuous at all times and write the complete analytical expression of this function.

Solve
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The following graph shows the concentration $C(t)$ of a drug in a patient's blood as a function of time $t$ (in hours). The curve for $0 \le t \le 4$ follows a quadratic…

The following graph shows the concentration $C(t)$ of a drug in a patient's blood as a function of time $t$ (in hours). The curve for $0 \le t \le 4$ follows a quadratic model of the form $C(t) = at^2+bt$, and for $t > 4$ it follows an exponential model $C(t) = k \cdot 2^{-0.5t}$.

  1. a) Determine the values of the parameters $a$, $b$ and $k$ by observing the key points on the graph.
  2. b) Study the continuity of the function at $t=4$ and interpret the meaning of the discontinuity in the medical context of the drug's administration.
Time (h) Concentration (mg/L) 2 4 6 10 20
Solve
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The graph shows the representation of the function $g(x) = \frac{x^2 + bx + c}{x^2 + dx + e}$ for all $x \neq -1$, additionally defining $g(-1) = 2$. a) From the…

The graph shows the representation of the function $g(x) = \frac{x^2 + bx + c}{x^2 + dx + e}$ for all $x \neq -1$, additionally defining $g(-1) = 2$.

  1. a) From the asymptotes and the hole observable in the graph, determine the values of the parameters $b, c, d$ and $e$.
  2. b) Study the continuity of $g(x)$ over its entire domain and classify its discontinuities.
X Y 1 2 3 −1 −2 1 2 −1 g(x)
Solve

The Intermediate Value Theorem

If you drive from sea level up to 1,000 ft without teleporting, you must have passed 500 ft at some moment. Continuity forbids skipping values.

Some problems call the zero case Bolzano's theorem. It is the same thing.

A continuous curve that starts below the x-axis at x = a and ends above it at x = b must cross the x-axis at some point c between them. a b f(a) < 0 f(b) > 0 c
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Use Bolzano's theorem to prove that the equation $x^3 - 3x + 1 = 0$ has at least one real solution in the interval $[0, 1]$. Explain why the theorem is applicable to…

Use Bolzano's theorem to prove that the equation $x^3 - 3x + 1 = 0$ has at least one real solution in the interval $[0, 1]$. Explain why the theorem is applicable to this particular case.

Solve
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Determine for which values of the parameter $m$ it can be guaranteed, by rigorously applying Bolzano's theorem, that the function $f(x) = x^3 - mx^2 + 2$ has at least…

Determine for which values of the parameter $m$ it can be guaranteed, by rigorously applying Bolzano's theorem, that the function $f(x) = x^3 - mx^2 + 2$ has at least one root in the closed interval $[-1, 1]$.

Solve

Summary

See all the exercises in this section