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Derivatives · Derivatives

Application to the calculation of limits. L'Hôpital's rule

In 1st Bachillerato you already calculated limits with a $\frac{0}{0}$ indeterminate form by factorising or rationalising. With trigonometric, exponential or logarithmic functions those tricks are no longer enough: $\lim_{x\to 0}\frac{e^x-1}{\sin x}$ cannot be factorised. L'Hôpital's rule makes it possible to resolve many indeterminate forms of this type when its conditions are met, by replacing the quotient of functions with the quotient of their derivatives.

Before you start, make sure you know: Rules of differentiation (1st Bachillerato)
Before you start, make sure you know: Calculating limits of functions (1st Bachillerato)

The rule: a quotient of derivatives instead of a quotient of functions

Flowchart for applying l'Hôpital's rule: checking for a 0/0 or infinity/infinity indeterminate form, differentiating the numerator and denominator separately, and the criterion for repeating. Is lim f(x)/g(x) of the form 0/0 or ∞/∞? yes → differentiate numerator and denominator separately lim f'(x)/g'(x) Still 0/0 or ∞/∞? → repeat. If not, that is the limit.
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Calculate the following limit using L'Hôpital's Rule, first verifying that the necessary indeterminate form holds: $$\lim_{x \to 2} \frac{x^3 - 5x + 2}{x^2 - 4}$$

Calculate the following limit using L'Hôpital's Rule, first verifying that the necessary indeterminate form holds:

$$\lim_{x \to 2} \frac{x^3 - 5x + 2}{x^2 - 4}$$
Solve
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In the study of electromagnetic signal propagation, the following expression appears for calculating the intensity at a critical point. Find the value of the limit as…

In the study of electromagnetic signal propagation, the following expression appears for calculating the intensity at a critical point. Find the value of the limit as time $t$ tends to $0$:

$$\lim_{t \to 0} \frac{e^{3t} - 1}{\sin(2t)}$$
Solve
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Determine the value of the following limit of a logarithmic function. Remember to correctly differentiate the numerator and the denominator independently: $$\lim_{x \to…

Determine the value of the following limit of a logarithmic function. Remember to correctly differentiate the numerator and the denominator independently:

$$\lim_{x \to 1} \frac{\ln(x)}{x^2 + 2x - 3}$$
Solve

$\infty/\infty$ indeterminate forms and repeated application

The rule works in the same way when the numerator and denominator both tend to infinity, and it may need more than one differentiation if the result is still indeterminate.

Graph of the function f(x) = x^2 / e^x between 0 and 10, showing how the curve reaches a maximum and decreases towards zero because the exponential dominates the power maximum at x = 2 tends to 0 (eˣ dominates x²)
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Calculate the limit of the following rational function at infinity. Although you could solve it by comparing degrees, use L'Hôpital's Rule to prove the result: $$\lim_{x…

Calculate the limit of the following rational function at infinity. Although you could solve it by comparing degrees, use L'Hôpital's Rule to prove the result:

$$\lim_{x \to +\infty} \frac{5x^2 - 3x + 1}{2x^2 + 7}$$
Solve
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In the study of the propagation of a wifi signal in an environment with interference, the loss of intensity has been modeled by the following function of the distance…

In the study of the propagation of a wifi signal in an environment with interference, the loss of intensity has been modeled by the following function of the distance $x$ (in meters):

$$f(x) = \frac{e^{2x} - e^{-2x} - 4x}{x - \sin(x)}$$

Calculate the limit of this function as the distance $x$ tends to $0$, that is, $\lim_{x \to 0} f(x)$, in order to determine the behavior of the signal at the emission point.

Solve
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We wish to study the convergence of an artificial intelligence algorithm. The error rate as a function of the number of iterations $x$ is defined as: $$E(x) =…

We wish to study the convergence of an artificial intelligence algorithm. The error rate as a function of the number of iterations $x$ is defined as:

$$E(x) = \frac{\ln(x^4 + 1)}{x}$$

Using L'Hôpital's Rule, show that as the number of iterations tends to infinity ($x \to \infty$), the error rate tends to $0$. What does this tell us about the long-term efficiency of the algorithm?

0 E(x) = ln(x⁴ + 1) / x x → ∞ E(x) → 0 Error Rate E(x) Iterations (x)
Solve

Unknown parameters from the value of the limit

A very typical exam question: the function has unknown letters $a, b$, and the value (or the existence) of the limit is given as data in order to find them. You differentiate treating the parameters as if they were numbers and equate coefficients.

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A team of sound engineers is analyzing the response of an audio filter. The gain of the filter as a function of the frequency $x$ is given by the expression: $$G(x) =…

A team of sound engineers is analyzing the response of an audio filter. The gain of the filter as a function of the frequency $x$ is given by the expression:

$$G(x) = \frac{a \cdot \cos(x) + b \cdot e^x - 2}{x^2}$$

Knowing that the limit of the gain as the frequency approaches zero is equal to $-1$, that is, $\lim_{x \to 0} G(x) = -1$, calculate with reasoning the values of the parameters $a$ and $b$.

Solve
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In the design of a new roller coaster, the curvature of an initial section is determined by the existence of the limit of a transition function. For the design to be…

In the design of a new roller coaster, the curvature of an initial section is determined by the existence of the limit of a transition function. For the design to be feasible, the following limit must be a finite real number:

$$\lim_{x \to 0} \frac{a \cdot \cos(x) + b \cdot e^x - 3 + x}{x^2}$$

Calculate, giving reasons, the values of the parameters $a, b \in \mathbb{R}$ so that this limit exists and is finite. Once obtained, calculate the value of the limit.

Solve

Other indeterminate forms: turning them into $0/0$ or $\infty/\infty$

The indeterminate forms $\infty-\infty$, $0\cdot\infty$, $1^\infty$, $0^0$ and $\infty^0$ cannot be differentiated directly: first they must be transformed into a quotient.

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Calculate the value of the following limit by first transforming the indeterminate form of type $\infty - \infty$ into a quotient in order to apply L'Hôpital's Rule:…

Calculate the value of the following limit by first transforming the indeterminate form of type $\infty - \infty$ into a quotient in order to apply L'Hôpital's Rule:

$$\lim_{x \to 1} \left( \frac{x}{x-1} - \frac{1}{\ln(x)} \right)$$
Solve
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Determine the value of the following limit, which presents an indeterminate form of type $1^\infty$. To solve it, use the properties of logarithms and L'Hôpital's Rule:…

Determine the value of the following limit, which presents an indeterminate form of type $1^\infty$. To solve it, use the properties of logarithms and L'Hôpital's Rule:

$$\lim_{x \to 0} (\cos x)^{\frac{1}{x^2}}$$ x y −1 1 1,0 0,5 0 x→0⁻ x→0⁺ L = ? f(x) = (cos x)1/x² Indeterminate form 1∞
Solve
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In a biotechnology laboratory, the growth of a bacterial population in a controlled environment follows a law whose cumulative accumulation rate is expressed by an…

In a biotechnology laboratory, the growth of a bacterial population in a controlled environment follows a law whose cumulative accumulation rate is expressed by an integral. We wish to study the behaviour of the population density at the initial instant by calculating the following limit:

$$\lim_{x \to 0} \frac{\int_{0}^{x} \ln(1 + t^2) \, dt}{x^3}$$

Use L'Hôpital's Rule and the Fundamental Theorem of Calculus to determine the value of this limit.

x → 0 y t 0 x ∫₀ˣ ln(1+t²)dt f(t) = ln(1+t²)
Solve

Summary

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