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Real numbers · Sequences

Limit of a Sequence

Before you start, make sure you know: Sequences of real numbers (Secondary Year 3)
Before you start, make sure you know: Properties of radicals and rationalization (High School Year 1 (semesters 1 and 2))

Ximena makes a cup of coffee at $85\,^\circ\text{C}$ and leaves it on a kitchen table at $20\,^\circ\text{C}$. Each minute it cools a little less than the minute before. Will it ever reach exactly $20\,^\circ\text{C}$? Or will it just get closer and closer? That question is the idea behind the limit of a sequence, and we answer it at the end of the page.

The intuition: getting close without arriving

Look at $a_n = \dfrac{n+1}{n}$: $2,\ 1.5,\ 1.333\ldots,\ 1.25,\ 1.2,\ \ldots$ The terms get closer to $1$, even though none of them is exactly $1$.

Terms of the sequence (n+1)/n shown as points that get closer and closer to the horizontal line L = 1 without touching it L = 1 the terms get close to 1 without touching it

Change the general term and watch which value (if any) the terms approach as $n$ increases:

Not every sequence approaches a number. If it does, we say it is convergent. If the terms grow without bound toward $+\infty$ or decrease toward $-\infty$, it is divergent. If it jumps around without approaching any value, it is oscillating and has no limit.

For example, $a_n = \dfrac{(-1)^n \cdot 3}{n}$ gives $-3,\ 1.5,\ -1,\ 0.75,\ \ldots$ It changes sign, but its absolute value $\frac{3}{n}$ tends to $0$. It approaches $0$ from both sides, so it converges to $0$.

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Determine whether the sequence whose general term is $a_n = \frac{(-1)^n \times 5}{n}$ is convergent, divergent or oscillating. If it is convergent, state its limit.

Determine whether the sequence whose general term is $a_n = \frac{(-1)^n \times 5}{n}$ is convergent, divergent or oscillating. If it is convergent, state its limit.

Solve

How close? A numerical look

"As close as we like" can also be explored with numbers: we fix a small distance and look for the term from which the sequence is closer than that to its limit.

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Given the sequence $a_n = \frac{4n - 3}{n + 1}$, we know that its limit is $L = 4$. Using the formal definition of a limit, find from which term $n$ onwards the distance…

Given the sequence $a_n = \frac{4n - 3}{n + 1}$, we know that its limit is $L = 4$. Using the formal definition of a limit, find from which term $n$ onwards the distance between the terms of the sequence and its limit is strictly less than one thousandth, that is, $|a_n - 4| < 0.001$.

Solve

Limits of rational sequences

The most common case is a fraction of polynomials. The result depends on comparing the degrees.

Diagram with the three possible results for the limit of a quotient of polynomials depending on their degrees degree P < degree Qlimit 0 degree P = degree Qquotient of leading coeff. degree P > degree Q±∞ (divergent)
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Find the limit of the rational sequence given by the expression: $c_n = \frac{3n^2 - 5n + 1}{8n^3 + 2n^2 - n}$

Find the limit of the rational sequence given by the expression:

$c_n = \frac{3n^2 - 5n + 1}{8n^3 + 2n^2 - n}$

Solve
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Calculate the limit of the following sequence, first identifying the degrees of the numerator and denominator: $d_n = \frac{6n^2 - 3n + 5}{2n^2 + 4n - 1}$

Calculate the limit of the following sequence, first identifying the degrees of the numerator and denominator:

$d_n = \frac{6n^2 - 3n + 5}{2n^2 + 4n - 1}$

Solve
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The fish population of a sustainable fish farm grows according to the mathematical model $P_n = \frac{150000n}{3n + 250}$, where $n$ represents the number of days since…

The fish population of a sustainable fish farm grows according to the mathematical model $P_n = \frac{150000n}{3n + 250}$, where $n$ represents the number of days since the introduction of the fingerlings. Determine the carrying capacity of the environment, that is, the long-term limit of the fish population.

Solve

Polynomials, different degrees, and special products

A polynomial by itself (with no denominator) is ruled by its highest-degree term: $\lim (-5n^3 + 2n^2 - n) = -\infty$, because $-5n^3$ dominates and is negative. If the numerator has a higher degree than the denominator, the sign of the quotient of leading coefficients decides between $+\infty$ and $-\infty$.

When the numerator comes with products or powers, expand first using special products (square of a binomial, conjugate binomials) to see the highest-degree term.

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Calculate the limit of the following polynomial sequence as $n$ tends to infinity: $b_n = -2n^3 + 7n^2 - 4n + 12$

Calculate the limit of the following polynomial sequence as $n$ tends to infinity:

$b_n = -2n^3 + 7n^2 - 4n + 12$

Solve
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Determine the value that the sequence $e_n$ tends to for very large values of $n$: $e_n = \frac{n^4 + 2n^2 - 7}{5n^2 - 3n + 1}$

Determine the value that the sequence $e_n$ tends to for very large values of $n$:

$e_n = \frac{n^4 + 2n^2 - 7}{5n^2 - 3n + 1}$

Solve
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Find the limit of the following sequence, first expanding the algebraic identities in the numerator to identify the terms of highest degree:…

Find the limit of the following sequence, first expanding the algebraic identities in the numerator to identify the terms of highest degree:

$\lim_{n \to \infty} \frac{(3n+2)^2 - (3n-2)^2}{n+5}$

Solve

Sequences with $r^n$: growth and cooling

A sequence $a_n = k\cdot r^n$ multiplies by $r$ at each step. Its limit depends on $r$.

Back to the beginning: the coffee never reaches exactly $20\,^\circ\text{C}$, but it gets as close as you like. That temperature is its limit.

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Study the behavior and calculate the limit of the exponential-type sequence: $f_n = 12 \times \left(\frac{3}{4}\right)^n$

Study the behavior and calculate the limit of the exponential-type sequence:

$f_n = 12 \times \left(\frac{3}{4}\right)^n$

Solve
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The temperature in degrees Celsius of a piece of metal cooling in a room follows the sequence $T_n = 22 + 85 \times (0,82)^n$, where $n$ is the time elapsed in minutes.…

The temperature in degrees Celsius of a piece of metal cooling in a room follows the sequence $T_n = 22 + 85 \times (0,82)^n$, where $n$ is the time elapsed in minutes. Calculate the limit of the sequence $T_n$ as $n \to \infty$ and interpret the result in the context of the problem.

Solve

The indeterminate forms $\infty-\infty$ and radicals

A subtraction of two quantities that tend to infinity is indeterminate. There are two tools: combine into a single fraction, or multiply and divide by the conjugate when there are roots.

A single root in a quotient is handled the same way: divide by $n$. For example, $\dfrac{\sqrt{9n^2+n}}{3n+1} = \dfrac{\sqrt{9+\frac{1}{n}}}{3+\frac{1}{n}} \to \dfrac{3}{3}=1$.

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Calculate the limit of the sequence involving a simple square root in the numerator: $h_n = \frac{\sqrt{16n^2 + 3n}}{2n + 5}$

Calculate the limit of the sequence involving a simple square root in the numerator:

$h_n = \frac{\sqrt{16n^2 + 3n}}{2n + 5}$

Solve
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Resolve the indeterminate form that arises when calculating the limit of the following difference of rational sequences:…

Resolve the indeterminate form that arises when calculating the limit of the following difference of rational sequences:

$\lim_{n \to \infty} \left( \frac{n^2 + 5n}{n + 2} - n \right)$

Solve
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Calculate the limit of the following irrational sequence, multiplying and dividing by the conjugate expression to resolve the indeterminate form:…

Calculate the limit of the following irrational sequence, multiplying and dividing by the conjugate expression to resolve the indeterminate form:

$\lim_{n \to \infty} \left( \sqrt{n^2 + 6n} - n \right)$

Solve

Conjugates with an extra factor and parameters

Sometimes the conjugate leaves a factor that keeps growing, or there are letters you must determine.

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Calculate the limit of the following sequence, which features a double square root and requires careful application of the conjugate technique:…

Calculate the limit of the following sequence, which features a double square root and requires careful application of the conjugate technique:

$\lim_{n \to \infty} n \times \left( \sqrt{n^2 + 4} - n \right)$

Solve
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Find the values of the real parameters $a$ and $b$ (with $b > 0$) such that the following equality holds:…

Find the values of the real parameters $a$ and $b$ (with $b > 0$) such that the following equality holds:

$\lim_{n \to \infty} \left( \sqrt{9n^2 + an + 2} - bn \right) = 4$

Solve

The number $e$ and the indeterminate form $1^\infty$

If the base tends to $1$ and the exponent to infinity, the result is not $1$: $1^\infty$ is an indeterminate form. The basic case is the number $e \approx 2.71828$.

It is the number that the values $\left(1+\frac1n\right)^n$ approach: $2$, $2.25$, $2.37$, $2.59$ (with $n=10$), $2.7048$ ($n=100$).

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Applying the definition of the number $e$, calculate the limit of the following sequence: $g_n = \left(1 + \frac{1}{n}\right)^{7n}$

Applying the definition of the number $e$, calculate the limit of the following sequence:

$g_n = \left(1 + \frac{1}{n}\right)^{7n}$

Solve
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Calculate the limit of the sequence, transforming the expression to apply the limit definition of the number $e$:…

Calculate the limit of the sequence, transforming the expression to apply the limit definition of the number $e$:

$\lim_{n \to \infty} \left( \frac{n + 7}{n + 2} \right)^{4n}$

Solve
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An amount of $100\,000$ pesos is deposited into an investment fund offering an annual interest rate of $3.5\%$. If the institution compounds the interest $n$ times a…

An amount of $100\,000$ pesos is deposited into an investment fund offering an annual interest rate of $3.5\%$. If the institution compounds the interest $n$ times a year, the capital accumulated after $10$ years is given by the sequence $C_n = 100\,000 \times \left(1 + \frac{0.035}{n}\right)^{10n}$. Calculate the limiting capital that would be obtained if compounding were continuous ($n \to \infty$).

Solve

When the exponent grows faster than $a_n-1$ shrinks, the product $(a_n-1)b_n$ can go to $\pm\infty$. Then the result is $e^{+\infty}=+\infty$ or $e^{-\infty}=0$. Use this in the next exercise, computing $(a_n-1)b_n$ carefully.

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Solve the following limit that leads to an indeterminate form of the type $1^\infty$, paying special attention to the algebraic operations in the exponent:…

Solve the following limit that leads to an indeterminate form of the type $1^\infty$, paying special attention to the algebraic operations in the exponent:

$\lim_{n \to \infty} \left( \frac{n^2 - n + 3}{n^2 + n + 1} \right)^{n^2 - 1}$

Solve

Infinite sums

Can a sum of infinitely many numbers give a finite result? Yes. The sum of the first $n$ terms is another sequence (the sequence of partial sums), and its limit is the infinite sum.

For a geometric progression with first term $a_1$ and ratio $r$, the sum of $n$ terms is $S_n=a_1\dfrac{1-r^n}{1-r}$. If $|r|<1$, then $r^n\to0$.

This is the same phenomenon as Zeno's paradox: covering half the distance, then a quarter, then an eighth… adds up to $1$ in total.

If the factor is greater than $1$, the sum grows without bound. For example, if a length is multiplied by $\frac{4}{3}$ at each step, $\left(\frac43\right)^n\to\infty$.

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Starting from a square of area $A_1 = 400 \text{ cm}^2$, a sequence of squares is constructed by joining the midpoints of the sides of the previous square, as shown in…
A₁ A₂ A₃

Starting from a square of area $A_1 = 400 \text{ cm}^2$, a sequence of squares is constructed by joining the midpoints of the sides of the previous square, as shown in the figure. Given that the area of each new square is half the area of the previous square, calculate the limit of the sum of the areas of the infinitely many squares thus constructed: $S_\infty = \lim_{n \to \infty} (A_1 + A_2 + \dots + A_n)$.

Solve
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The figure shows the first three iterations in the construction of the Koch curve. If we start from an initial segment of length $L_0 = 1$ meter (panel A), at each step…
A n = 0 B n = 1 C n = 2

The figure shows the first three iterations in the construction of the Koch curve. If we start from an initial segment of length $L_0 = 1$ meter (panel A), at each step $n$ the total length of the curve is multiplied by $\frac{4}{3}$ compared to the previous step. Write the expression for the length $L_n$ at step $n$ and calculate $\lim_{n \to \infty} L_n$. What geometric conclusion can be drawn from this result?

Solve
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A high-tech elastic cable undergoes iterative tension tests. Its initial length at rest is $20$ meters. In the first test it stretches $4$ meters. In each successive…

A high-tech elastic cable undergoes iterative tension tests. Its initial length at rest is $20$ meters. In the first test it stretches $4$ meters. In each successive test, the material loses elasticity and stretches by $75\%$ of the amount it stretched in the immediately preceding test. The total length after $n$ tests is given by $L_n = 20 + \sum_{k=1}^n 4 \times (0,75)^{k-1}$. Calculate the theoretical maximum length that the cable will reach if subjected to infinitely many tests.

Solve

Recurrent sequences and their limit

A sequence can be defined with a recurrence: each term is obtained from the previous one. If it converges to $L$, then $a_n\to L$ and also $a_{n+1}\to L$. Taking the limit in the formula gives an equation for $L$.

This also applies to models of medication dosage. If each day a fraction $q$ of the previous amount remains and a fixed amount is added, the equilibrium concentration is the fixed point $L$.

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A patient receives a daily dose of a medication. After absorption and elimination throughout the day, the concentration of the drug in the blood just before the next…

A patient receives a daily dose of a medication. After absorption and elimination throughout the day, the concentration of the drug in the blood just before the next dose follows the recurrence relation $C_n = 15 + C_{n-1} \times 0.4$, where $15 \text{ mg/L}$ is the contribution of the new dose and the factor $0.4$ indicates that $40\%$ of the previous day's drug remains. If the initial concentration is $C_1 = 15$, calculate the long-term equilibrium concentration in the blood, that is, $\lim_{n \to \infty} C_n$.

Solve
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A sequence is defined by the following recurrence relation: the first term is $a_1 = \sqrt{6}$, and each subsequent term is obtained as $a_{n+1} = \sqrt{6 + a_n}$.…

A sequence is defined by the following recurrence relation: the first term is $a_1 = \sqrt{6}$, and each subsequent term is obtained as $a_{n+1} = \sqrt{6 + a_n}$. Assuming that the sequence is increasing and bounded above (and is therefore convergent and has a limit $L$), set up the equation that the limit must satisfy and find it.

Solve

Summary

See all the exercises in this section