Exponential Sequences. The Number e
A commercial bank pays you interest more and more times a year: quarterly, monthly, daily, every second… How much more can you earn? The answer has a very specific ceiling, and that ceiling is one of the most important numbers in mathematics: the number $e$.
The Sequence That Defines the Number $e$
Calculate $\left(1+\dfrac{1}{n}\right)^n$ for larger and larger $n$: $n=1\to2$; $n=10\to2.5937\ldots$; $n=100\to2.7048\ldots$; $n=10{,}000\to2.71814\ldots$
Notice how the sequence approaches $e$ without ever exceeding it:
The result is a balance between two forces: the base $1+\frac1n$ gets closer to $1$, but the exponent $n$ keeps growing without bound.
Calculate the limit of the following exponential sequence: $a_n = \left( 1 + \frac{1}{n} \right)^{3n}$
Calculate the limit of the following exponential sequence:
$a_n = \left( 1 + \frac{1}{n} \right)^{3n}$
Find the limit of the sequence formed by the quotient of polynomials raised to $n$: $c_n = \left( \frac{n+1}{n} \right)^n$
Find the limit of the sequence formed by the quotient of polynomials raised to $n$:
$c_n = \left( \frac{n+1}{n} \right)^n$
Calculate the limit of the sequence with general term: $e_n = \left( 1 - \frac{1}{n} \right)^n$
Calculate the limit of the sequence with general term:
$e_n = \left( 1 - \frac{1}{n} \right)^n$
Variants of the Sequence for $e$
When the exponent is a multiple or the base changes slightly, it reduces to $e$ with a simple idea: $\left(1+\frac1n\right)^{3n}=\left[\left(1+\frac1n\right)^n\right]^3\to e^3$.
Determine the value that the sequence $b_n$ tends to as $n \to \infty$: $b_n = \left( 1 + \frac{1}{2n} \right)^{2n}$
Determine the value that the sequence $b_n$ tends to as $n \to \infty$:
$b_n = \left( 1 + \frac{1}{2n} \right)^{2n}$
Find the limit of the exponential sequence whose exponent is $n+5$: $g_n = \left( 1 + \frac{1}{n} \right)^{n+5}$
Find the limit of the exponential sequence whose exponent is $n+5$:
$g_n = \left( 1 + \frac{1}{n} \right)^{n+5}$
Calculate the limit of the sequence: $h_n = \left( \frac{n}{n+1} \right)^n$
Calculate the limit of the sequence:
$h_n = \left( \frac{n}{n+1} \right)^n$
The General Formula for $1^{\infty}$
The same idea repeats with many bases and exponents. There is a formula that turns any $1^{\infty}$ into a simpler limit: exponent times (base minus 1).
Determine the limit of the following sequence, given that it presents an indeterminate form of the type $1^\infty$: $f_n = \left( 1 + \frac{4}{n} \right)^n$
Determine the limit of the following sequence, given that it presents an indeterminate form of the type $1^\infty$:
$f_n = \left( 1 + \frac{4}{n} \right)^n$
Calculate the limit of the following sequence, resolving the corresponding indeterminate form: $a_n = \left( \frac{n+3}{n-1} \right)^{2n}$
Calculate the limit of the following sequence, resolving the corresponding indeterminate form:
$a_n = \left( \frac{n+3}{n-1} \right)^{2n}$
Determine the limit of the following sequence by resolving the indeterminate form of the type $1^\infty$: $b_n = \left( \frac{2n+5}{2n+1} \right)^{3n-2}$
Determine the limit of the following sequence by resolving the indeterminate form of the type $1^\infty$:
$b_n = \left( \frac{2n+5}{2n+1} \right)^{3n-2}$
More Complicated Exponents and Bases
Sometimes the exponent is an algebraic fraction or the base has degree-2 polynomials. The procedure is identical: compute $b_n(a_n-1)$ and solve that limit using degrees or factoring.
Find the limit of the sequence combining second-degree polynomials in the base: $c_n = \left( \frac{n^2+2n}{n^2-1} \right)^{5n}$
Find the limit of the sequence combining second-degree polynomials in the base:
$c_n = \left( \frac{n^2+2n}{n^2-1} \right)^{5n}$
Calculate the limit of the following sequence, which features an algebraic exponent in fractional form: $d_n = \left( \frac{n-2}{n+4} \right)^{\frac{n^2+1}{n}}$
Calculate the limit of the following sequence, which features an algebraic exponent in fractional form:
$d_n = \left( \frac{n-2}{n+4} \right)^{\frac{n^2+1}{n}}$
Calculate the limit of the exponential sequence formed by second-degree polynomials in both the base and the exponent:…
Calculate the limit of the exponential sequence formed by second-degree polynomials in both the base and the exponent:
$a_n = \left( \frac{2n^2-3n+1}{2n^2+n-5} \right)^{\frac{n^2+3}{n}}$
Find the limit of the sequence combining square roots in its base and a linear exponent: $c_n = \left( \sqrt{\frac{n+2}{n-1}} \right)^{3n+1}$
Find the limit of the sequence combining square roots in its base and a linear exponent:
$c_n = \left( \sqrt{\frac{n+2}{n-1}} \right)^{3n+1}$
Calculate the limit of the sequence by resolving the indeterminate form generated by the algebraic quotient in the exponent:…
Calculate the limit of the sequence by resolving the indeterminate form generated by the algebraic quotient in the exponent:
$d_n = \left( \frac{n^2+5n-2}{n^2+3n+1} \right)^{\frac{n^3}{n^2+1}}$
Parameters and Problems with $1^\infty$
If the limit is known and a parameter is missing, set the exponent of $e$ equal to the given value.
Find the value of the real parameter $a$ such that the following equality involving the number $e$ holds: $\lim_{n \to \infty} \left( \frac{n+a}{n+1} \right)^{2n} = e^6$
Find the value of the real parameter $a$ such that the following equality involving the number $e$ holds:
$\lim_{n \to \infty} \left( \frac{n+a}{n+1} \right)^{2n} = e^6$
Calculate the values of the parameters $a$ and $b$ such that the following sequence has limit $e^{-4}$, given that $a$ is a positive real number:…
Calculate the values of the parameters $a$ and $b$ such that the following sequence has limit $e^{-4}$, given that $a$ is a positive real number:
$\lim_{n \to \infty} \left( \frac{an+3}{2n+1} \right)^{bn} = e^{-4}$
The concentration of a pollutant in a lake is naturally reduced by a process that follows the sequence $C_n = C_0 \left( 1 - \frac{2}{n} \right)^n$, where $n$ represents…
The concentration of a pollutant in a lake is naturally reduced by a process that follows the sequence $C_n = C_0 \left( 1 - \frac{2}{n} \right)^n$, where $n$ represents the number of filtering stages applied. If infinitely many filtering stages are applied, prove mathematically to what exact fraction of the initial concentration $C_0$ the pollutant will be reduced.
The Number $e$ in Continuously Compounded Interest
If you compound a capital $C$ $n$ times a year at a nominal interest rate $r$, after one year you have $C\left(1+\dfrac{r}{n}\right)^n$. When $n\to\infty$ (continuous compounding), $e$ appears.
An amount of $300\,000$ pesos is deposited in a financial institution at an annual interest rate of $4\%$. The formula for the final capital after $n$ years with…
An amount of $300\,000$ pesos is deposited in a financial institution at an annual interest rate of $4\%$. The formula for the final capital after $n$ years with continuous compounding is given by the sequence $C_n = 300\,000 \times e^{0.04n}$. Calculate the accumulated capital after $10$ years and compare it with the capital obtained if the compounding were annual, whose expression is $C'_n = 300\,000 \times (1,04)^n$.
An amount of $600\,000$ pesos is invested at a nominal annual interest rate of $5\%$. The sequence describing the final capital if interest is compounded $n$ times a…
An amount of $600\,000$ pesos is invested at a nominal annual interest rate of $5\%$. The sequence describing the final capital if interest is compounded $n$ times a year is $C_n = 600\,000 \left( 1 + \frac{0.05}{n} \right)^n$. Calculate the limit of this sequence as $n \to \infty$ to find the final capital under continuous compounding. Compare this result with the capital obtained if compounded quarterly ($n=4$).
Exponential Growth and Decay
Many processes (radioactive decay, cooling, depreciation, medications) are modeled with an exponential: $a_n=a_0\,e^{kn}$ with $k>0$ for growth and $k<0$ for decay. Also with base $q$: $a_n=a_0q^n$ tends to $0$ if $01$.
State the limit of the following geometric sequence: $d_n = 5 \times \left( \frac{2}{3} \right)^n$
State the limit of the following geometric sequence:
$d_n = 5 \times \left( \frac{2}{3} \right)^n$
The amount of a radioactive isotope in a sample decreases exponentially according to the sequence $M_n = 200 \times e^{-0.1n}$, where $n$ is the number of years elapsed…
The amount of a radioactive isotope in a sample decreases exponentially according to the sequence $M_n = 200 \times e^{-0.1n}$, where $n$ is the number of years elapsed and $M_n$ is measured in grams. How many grams will remain in the sample after $20$ years? What value does the mass tend to in the long term (as $n \to \infty$)?
A vehicle loses $15\%$ of its value each year due to depreciation. If its initial price at the dealership is $24000$ pesos, its value after $n$ years is given by the…
A vehicle loses $15\%$ of its value each year due to depreciation. If its initial price at the dealership is $24000$ pesos, its value after $n$ years is given by the sequence $V_n = 24000 \times (0,85)^n$. What will its value be after $5$ years? What happens to its value as the number of years tends to infinity?
In a thermodynamics experiment, the temperature difference between a metallic body and its surroundings decreases exponentially according to the sequence…
In a thermodynamics experiment, the temperature difference between a metallic body and its surroundings decreases exponentially according to the sequence $T_n = 80 \times e^{-0.05n}$, where $n$ is the minutes elapsed since the start of cooling. Calculate the temperature difference after $30$ minutes and determine the limit of the sequence as $n \to \infty$.
A patient receives a dose of $50$ mg of a medication every $24$ hours. Given that the body eliminates $40\%$ of the medication present during that time interval, the…
A patient receives a dose of $50$ mg of a medication every $24$ hours. Given that the body eliminates $40\%$ of the medication present during that time interval, the quantity accumulated in the body immediately after the $n$th dose follows the geometric sequence $C_n = 50 + 50 \times 0.6 + 50 \times 0.6^2 + \dots + 50 \times 0.6^{n-1}$. Calculate the long-term accumulated quantity (the limit of the sequence as $n \to \infty$).
Back to the beginning: crediting interest continuously does not send your earnings off to infinity: the capital is bounded by $C\,e^r$. At $5\,\%$, the return rises from $5\,\%$ to a ceiling of $5.13\,\%$.