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Limits and continuity · Limits

Indeterminate Forms

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: Radicals (High School Year 2 (semesters 3 and 4))

Your cell phone charges very fast at first and more and more slowly as it nears the end. If the battery percentage were $B(t)=\dfrac{100\,t}{t+2}$ (with $t$ in hours), what value does the charge approach if you leave it plugged in for a very long time? Substituting $t\to\infty$ gives $\frac{\infty}{\infty}$, which tells you nothing. Let's see what to do in this case and in other similar ones.

$\dfrac{0}{0}$: factoring and the conjugate

This is the indeterminate form you already know: the numerator and denominator both vanish at $x=a$, so both have a "hidden" factor $(x-a)$. The technique consists of bringing it into view and canceling it.

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Calculate the limit of the function $f(x) = \frac{x^2 - 9}{x - 3}$ as $x$ tends to $3$. Help: Notice that direct substitution gives an indeterminate form of the type…

Calculate the limit of the function $f(x) = \frac{x^2 - 9}{x - 3}$ as $x$ tends to $3$.

Help: Notice that direct substitution gives an indeterminate form of the type $\left[\frac{0}{0}\right]$. Try factorising the numerator to simplify the expression.

Solve
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Calculate the following limit by resolving the indeterminate form $\frac{0}{0}$ using the conjugate technique: $$\lim_{x \to 4} \frac{x^2 - 16}{\sqrt{x+5} - 3}$$

Calculate the following limit by resolving the indeterminate form $\frac{0}{0}$ using the conjugate technique:

$$\lim_{x \to 4} \frac{x^2 - 16}{\sqrt{x+5} - 3}$$
Solve
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Solve the following limit using the double conjugate technique to resolve the $\frac{0}{0}$ indeterminate form: $$\lim_{x \to 0} \frac{\sqrt{1+x} -…

Solve the following limit using the double conjugate technique to resolve the $\frac{0}{0}$ indeterminate form:

$$\lim_{x \to 0} \frac{\sqrt{1+x} - \sqrt{1-x}}{\sqrt{4+x} - 2}$$
Solve
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A team of aerospace engineers is studying the pressure $P$ (in pascals) inside a ventilation duct. The pressure depends on the airflow speed $v$ according to the…

A team of aerospace engineers is studying the pressure $P$ (in pascals) inside a ventilation duct. The pressure depends on the airflow speed $v$ according to the formula:

$$P(v) = \frac{v^2 - 9}{\sqrt{v^2 + 7} - 4}$$

Study the behavior of the pressure as the speed $v$ approaches $3$ m/s. If the limit is indeterminate, resolve it and state whether the pressure tends to a stable value.

v ➞ 3 P(v) = v² - 9 √(v² + 7) - 4 P(3) ➞ ? [ 0/0 ] Ventilation duct
Solve

$\dfrac{\infty}{\infty}$: the dominant term

When the numerator and denominator grow without bound, the one that grows faster wins. In quotients of polynomials, you compare the degrees.

With this you can solve the opening problem: $\displaystyle\lim_{t\to+\infty}\frac{100t}{t+2}=\frac{100}{1}=100$. The charge approaches $100\%$.

If there are roots, remember that $\sqrt{x^n}$ counts as degree $\frac{n}{2}$ (and $\sqrt[3]{x^n}$ as $\frac{n}{3}$).

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Resolve the indeterminate form $\left[\frac{\infty}{\infty}\right]$ in the following limits of rational functions by comparing the degrees of the numerator and…

Resolve the indeterminate form $\left[\frac{\infty}{\infty}\right]$ in the following limits of rational functions by comparing the degrees of the numerator and denominator:

  1. a) $\lim_{x \to +\infty} \frac{5x^2 - 3x + 2}{2x^2 + 1}$
  2. b) $\lim_{x \to +\infty} \frac{x + 5}{x^2 - 3}$
  3. c) $\lim_{x \to +\infty} \frac{x^3 - 1}{x^2 + 10x}$
Solve
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A team of scientists is monitoring the growth of a population of bacteria in a laboratory. The population (in millions) follows the model given by the function: $$P(t) =…

A team of scientists is monitoring the growth of a population of bacteria in a laboratory. The population (in millions) follows the model given by the function:

$$P(t) = \frac{20t + 50}{t + 2}$$

where $t$ is the time in hours since the start of the experiment. The graph shows the evolution of the population during the first 20 hours.

What will the bacteria population be if the experiment were extended indefinitely? To find out, calculate $\lim_{t \to +\infty} P(t)$.

Graph of the bacteria population P(t) in millions during the first 20 hours: it starts at 25 million and decreases slowly t (hours) P(t) (millions) 0 10 20 30 4 8 12 16 20 P₀ = 25
Solve
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Determine the value of the limit at infinity by analysing the degree of the dominant terms in the numerator and the denominator. Take special care with the powers inside…

Determine the value of the limit at infinity by analysing the degree of the dominant terms in the numerator and the denominator. Take special care with the powers inside the roots:

$$\lim_{x \to +\infty} \frac{\sqrt{9x^2 + 2x - 1} + 2x}{\sqrt[3]{x^3 + 5x^2}}$$
Solve

$\dfrac{\infty}{\infty}$ with exponentials and logarithms: hierarchy

If $\ln x$, powers and exponentials appear in the quotient, comparing degrees is no longer enough. You use the hierarchy of infinities.

Hierarchy of infinities: the logarithm grows more slowly than powers, powers more slowly than exponentials, and exponentials more slowly than the factorial. ln x xk ax n! grows faster and faster → with a > 1 and k > 0
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In the study of computer algorithms, it is crucial to know which function grows faster. Based on the hierarchy of infinities, assign the value of the following limits…

In the study of computer algorithms, it is crucial to know which function grows faster. Based on the hierarchy of infinities, assign the value of the following limits ($0$, $+\infty$ or $-\infty$):

  1. a) $\lim_{x \to +\infty} \frac{\ln(x)}{x}$
  2. b) $\lim_{x \to +\infty} \frac{e^x}{x^{10}}$
  3. c) $\lim_{x \to +\infty} \frac{x^2 + 1}{\ln(x)}$
Solve
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Use comparison of orders of infinity to solve the following limits without performing lengthy calculations. Justify your answer by indicating which function "dominates"…

Use comparison of orders of infinity to solve the following limits without performing lengthy calculations. Justify your answer by indicating which function "dominates" the other:

  1. a) $\lim_{x \to +\infty} \frac{e^x + x^5}{100x^6 - \ln(x)}$
  1. b) $\lim_{x \to +\infty} \frac{\sqrt{x^7 + 1}}{x^3 \times \ln(x)}$
Solve
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Calculate the value of the following limit by analysing the hierarchy of the infinities involved. Give reasons to justify which term dominates in the numerator and in…

Calculate the value of the following limit by analysing the hierarchy of the infinities involved. Give reasons to justify which term dominates in the numerator and in the denominator before providing the final result:

$$\lim_{x \to +\infty} \frac{3 \times e^{2x} - \ln(x^5 + 1) + x^{10}}{2 \times 5^x + \sqrt{x^{20} - 3x^2} - e^x}$$ x → +∞ y 0 Exponencial (aˣ) e.g.: 3·e²ˣ, 2·5ˣ, eˣ Polinómica (xⁿ) e.g.: x¹⁰, √(x²⁰-3x²) Logarítmica (ln x) e.g.: ln(x⁵+1) lim x→+∞ 3·e²ˣ - ln(x⁵+1) + x¹⁰ 2·5ˣ + √(x²⁰-3x²) - eˣ = ?
Solve

$\infty-\infty$: fractions, conjugate and roots of different index

Here there are three cases:

  • Fractions (limit at a point): subtract them with a common denominator and almost always a $\frac00$ appears that you factor.

  • A square root minus something (limit at $\infty$): multiply and divide by the conjugate.

  • Roots of different index (square and cube), extension: subtract and add an intermediate term and treat each difference with its own conjugate.

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Resolve the following indeterminate form $\infty - \infty$. To do so, perform the preliminary algebraic operation and simplify the resulting expression: $$\lim_{x \to 1}…

Resolve the following indeterminate form $\infty - \infty$. To do so, perform the preliminary algebraic operation and simplify the resulting expression:

$$\lim_{x \to 1} \left( \frac{6}{x^2 - 1} - \frac{3}{x - 1} \right)$$
Solve
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Calculate the value of the following expression which presents an indeterminate form of the type $\infty - \infty$ with roots of different orders: $$\lim_{x \to +\infty}…

Calculate the value of the following expression which presents an indeterminate form of the type $\infty - \infty$ with roots of different orders:

$$\lim_{x \to +\infty} \left[ \sqrt{x^2 + 4x} - \sqrt[3]{x^3 - 6x^2} \right]$$
Solve

$1^\infty$: the formula for the number $e$

When the base approaches $1$ and the exponent grows without bound in absolute value (to $+\infty$ or to $-\infty$), the result is a power of $e$.

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Calculate the value of the following limit leading to an indeterminate form of the type $[1^\infty]$ using the shortcut formula for the number $e$: $$\lim_{x \to…

Calculate the value of the following limit leading to an indeterminate form of the type $[1^\infty]$ using the shortcut formula for the number $e$:

$$\lim_{x \to +\infty} \left( 1 + \frac{1}{x} \right)^{3x}$$
Solve
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Calculate the value of the following exponential limit which leads to an indeterminate form of the type $1^\infty$. Express the result in terms of the number $e$:…

Calculate the value of the following exponential limit which leads to an indeterminate form of the type $1^\infty$. Express the result in terms of the number $e$:

$$\lim_{x \to 0} (1 + \sin(x))^{\frac{2}{x}}$$
Solve

$1^\infty$ with quotients and parameters

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In the development of a new social network, the number of active users (in millions) follows the function $U(t) = \left(\frac{2t+3}{2t-1}\right)^{t+1}$, where $t$ is the…

In the development of a new social network, the number of active users (in millions) follows the function $U(t) = \left(\frac{2t+3}{2t-1}\right)^{t+1}$, where $t$ is the time in months since its launch (with $t>\frac{1}{2}$). Calculate the value the number of users approaches as time grows indefinitely ($t \to +\infty$).

Solve
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In the design of a new telecommunications antenna, the signal intensity $I$ as a function of the distance $d$ to the repeater is given by the expression: $$I(d) = \left(…

In the design of a new telecommunications antenna, the signal intensity $I$ as a function of the distance $d$ to the repeater is given by the expression:

$$I(d) = \left( \frac{d^2 + 5d}{d^2 - 2d + 1} \right)^{2d + 3}$$

Calculate the limiting signal intensity as distance $d$ tends to infinity (indeterminate form of the type $1^\infty$).

Solve

Parameters: what values must $a$ and $b$ have

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Find the values of the real parameters $a$ and $b$ so that the following limit is finite and equals $4$: $$\lim_{x \to +\infty} \sqrt{x^2 + ax + 1} - \sqrt{x^2 + bx}$$

Find the values of the real parameters $a$ and $b$ so that the following limit is finite and equals $4$:

$$\lim_{x \to +\infty} \sqrt{x^2 + ax + 1} - \sqrt{x^2 + bx}$$
Solve
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Determine the values of the real parameters $a$ and $b$ so that the following equality holds: $$\lim_{x \to 2} \frac{\sqrt{x^2 + ax + b} - 2}{x^2 - 5x + 6} =…

Determine the values of the real parameters $a$ and $b$ so that the following equality holds:

$$\lim_{x \to 2} \frac{\sqrt{x^2 + ax + b} - 2}{x^2 - 5x + 6} = -\frac{1}{2}$$

That is, the limit must be a finite real number and equal $-\dfrac{1}{2}$. Once you have found $a$ and $b$, check that the limit really equals $-\dfrac{1}{2}$.

Solve

One-sided limits, piecewise functions and discontinuities

In a piecewise function, the limit at the joining point is calculated separately on each side. The overall limit exists only if the two one-sided limits coincide.

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Given the following graph of the function $f(x)$, analyse visually and calculate analytically the one-sided limits at the points of discontinuity to determine whether…

Given the following graph of the function $f(x)$, analyse visually and calculate analytically the one-sided limits at the points of discontinuity to determine whether the overall limit exists at $x = 2$:

$$f(x) = \begin{cases} \frac{x^2 - 4}{x - 2} & \text{if } x < 2 \\ ax - 1 & \text{if } x \geq 2 \end{cases}$$

Calculate the value of $a$ so that the limit at $x = 2$ is unique and the function does not have a jump.

x y 0 Jump y = (x² - 4)/(x - 2) y = ax - 1 2 2 4
Solve
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Let the function be defined piecewise: $$f(x) = \begin{cases} \frac{x^2 - kx}{x - \sqrt{x}} & \text{if } x > 1 \\ \frac{a \times \sin(x-1)}{x^2 - 1} & \text{if } x < 1…

Let the function be defined piecewise:

$$f(x) = \begin{cases} \frac{x^2 - kx}{x - \sqrt{x}} & \text{if } x > 1 \\ \frac{a \times \sin(x-1)}{x^2 - 1} & \text{if } x < 1 \end{cases}$$

Find the values of $a$ and $k$ so that the one-sided limits at $x=1$ exist and are equal to $2$. What type of discontinuity does the function have at $x=1$ if we do not define $f(1)$?

Solve
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Given the function $g(x) = \frac{|x-1|}{x^2-1} \times e^{\frac{1}{x-1}}$, analyse the existence of the one-sided limits as $x \to 1$. Identify the indeterminate forms…

Given the function $g(x) = \frac{|x-1|}{x^2-1} \times e^{\frac{1}{x-1}}$, analyse the existence of the one-sided limits as $x \to 1$. Identify the indeterminate forms that appear on each side and classify the type of discontinuity at that point.

Solve

Summary

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