Name: improper integrals and exponential decay
Uploaded: 2026-01-12T12:25:00.000Z
Duration: 1 min 29 s
Channel: JoVE

Une intégrale est considérée comme impropre en raison d’un intervalle infini lorsque la limite supérieure ou inférieure d’intégration s’étend à l’infini, créant une région non bornée sous la courbe.Dans ce cas, la borne infinie est remplacée par une variable, et l’intégrale est évaluée en prenant la limite lorsque cette variable tend vers l’infini.Cette méthode aide à déterminer si l’aire totale sous la courbe reste finie même sur un domaine infini.Un exemple pratique consiste à calculer l’intensité totale intégrée de la lumière qui traverse un milieu, comme le brouillard, sur une distance infinie, en supposant que le milieu soit uniforme.Dans de tels cas, l’intensité lumineuse diminue avec la distance, suivant un motif de désintégration exponentielle.Pour trouver l’intensité intégrée totale, la limite supérieure infinie est d’abord remplacée par une variable, t.La fonction d’intensité est alors intégrée de zéro à t, et les limites d’intégration sont substituées pour donner une expression incluant un terme exponentiel.La dernière étape consiste à prendre la limite lorsque t tend vers l’infini, où le terme exponentiel tend vers zéro, laissant une valeur finie.Cela confirme que l’intensité totale intégrée — l’aire sous la courbe — peut rester finie même avec un domaine infini.

intégrales impropres : intervalles infinis

Une int&#xE9;grale est dite impropre lorsque, au moins, une de ses bornes d&#x2019;int&#xE9;gration tend vers l&#x2019;infini positif ou n&#xE9;gatif. Dans ce cas, la r&#xE9;gion sous la courbe est non born&#xE9;e, et les techniques classiques d&#x2019;&#xE9;valuation des int&#xE9;grales d&#xE9;finies ne sont pas directement applicables. L&#x2019;int&#xE9;grale impropre est d&#xE9;finie par un processus de passage &#xE0; la limite qui permet de d&#xE9;terminer si l&#x2019;aire accumul&#xE9;e reste finie malgr&#xE9; l&#x2019;infinit&#xE9; du domaine.Application aux mod&#xE8;les de d&#xE9;croissance exponentielleUne application pratique des int&#xE9;grales impropres sur des intervalles infinis consiste &#xE0; mod&#xE9;liser des processus de d&#xE9;croissance exponentielle. On peut citer comme exemple l&#x2019;intensit&#xE9; lumineuse totale int&#xE9;gr&#xE9;e traversant un milieu uniforme, tel que le brouillard, sur une distance infinie. Dans ce cas, l&#x2019;intensit&#xE9; lumineuse d&#xE9;cro&#xEE;t exponentiellement avec la distance et peut &#xEA;tre mod&#xE9;lis&#xE9;e par une fonction de la forme I(x) = I_0 e^{-kx}, o&#xF9; I_0 est l&#x2019;intensit&#xE9; initiale et k &gt; 0 est une constante de d&#xE9;croissance.Pour calculer l&#x2019;intensit&#xE9; int&#xE9;gr&#xE9;e totale, l&#x2019;int&#xE9;grale\begin{equation*}\int_0^{\infty} I_0 e^{-kx}\,dx\end{equation*}est d&#x2019;abord r&#xE9;&#xE9;crite sous la forme\begin{equation*}\lim_{t \to \infty} \int_{0}^{t} I_0 e^{-kx}\,dx\end{equation*}L&#x2019;&#xE9;valuation de l&#x2019;int&#xE9;grale donne une expression contenant un terme exponentiel e^(&#x2212;kt). Lorsque t tend vers l&#x2019;infini, ce terme tend vers z&#xE9;ro, ce qui donne un r&#xE9;sultat fini. Cela montre que, m&#xEA;me si la distance est infinie, l&#x2019;intensit&#xE9; int&#xE9;gr&#xE9;e totale reste finie en raison de la d&#xE9;croissance exponentielle de la fonction.

Intégrales impropres : intervalles infinis

Une int&#xE9;grale est dite impropre lorsque, au moins, une de ses bornes d&#x2019;int&#xE9;gration tend vers l&#x2019;infini positif ou n&#xE9;gatif. ...

Education

Enseignement

JoVE Core

Calculus

Calcul

Unit

Unité

Techniques of Integration

Techniques d'intégration

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

Improper Integrals: Infinite Intervals

improper integrals and exponential decay

Improper integrals with infinite intervals appear when one limit of integration goes to positive or negative infinity. In that case, the area under the curve is unbounded, so ordinary definite integral methods do not apply directly. The integral is then defined by a limit, which helps show whether the total area is finite even when the interval is infinite.

This idea is useful in exponential decay models. One example is the total integrated intensity of light moving through a uniform medium, such as fog over an infinite distance. The light intensity can be written as I0e−kx, where I0 is the initial intensity and k is a positive decay constant.

To find the total integrated intensity, the integral is rewritten as a limit and evaluated. The result includes an exponential term, e−kt. As t approaches infinity, that term approaches zero. This leaves a finite total, showing that an infinite distance can still give a finite accumulated intensity when the function decays exponentially.

Improper Integrals and Exponential Decay

integration by parts: indefinite integrals

integration-by-parts-indefinite-integrals

Integration by Parts in AC Circuit Analysis

Integration by parts is a calculus method for handling integrals that contain a product of two functions. It is useful when direct integration is not practical. The method comes from the product rule for differentiation, which says that the derivative of a product is the derivative of the first function times the second, plus the first function times the derivative of the second. By integrating that product-rule identity and rearranging the terms, the integration by parts formula is obtained.

integration by parts: definite integrals

Using Parts and Substitution for Area

Integration by parts can make a definite integral easier to solve when the integrand is a product over a fixed interval. The method rewrites the problem as a product at the endpoints minus another definite integral that may be simpler to evaluate. A useful example is the definite integral of inverse tangent, or arctan x. Since there is no direct integration formula for arctan x, the integrand is written as arctan x times the constant function 1. The inverse tangent is chosen to differentiate,...

integration by parts: problem solving

Integration by Parts for Signal Processing

Integration by parts can help evaluate piecewise integrals that appear in signal processing. In smart speakers, audio inputs are modeled as piecewise functions and analyzed with trigonometric functions such as cosine. This lets complex sound waves be broken into simpler frequency parts. When a definite integral uses a piecewise function, the integral is split into separate parts. One part may contain a constant times cosine, which can be found with basic integration rules. Another part may...

integrals of powers of sine and cosine

Solving Trig Integrals with Odd and Even Powers

Trigonometric integrals are solved by looking at the powers of sine and cosine in the expression. These integrals appear often in calculus, and they can also be useful in physics and engineering. For integrals of the form sin^m(x)cos^n(x), the first step is to check whether the sine power or the cosine power is odd. If the sine power is odd, one sine factor is separated from the integrand. That leaves an even power of sine. The remaining sine terms are rewritten using the Pythagorean identity,...

integrals of powers of secant and tangent

integrals-of-powers-of-secant-and-tangent

Solving Secant and Tangent Integrals

Integrals of secant and tangent powers are often solved by choosing a trig substitution that matches the integrand. The key idea is to use the parity of the exponents, which means whether a power is even or odd. A good substitution lets part of the integrand pair with the derivative of a trig function. When the power of secant is even, tangent is the better substitution variable. Since the derivative of tangent is secant squared, a factor of sec^2 x can be split off. The remaining even power...

trigonometric substitution

Finding Ellipse Area with Trig Substitution

Trigonometric substitution is a method for solving integrals that contain square root expressions from quadratic forms. It works well when the integrand looks like a familiar geometric equation, such as a circle or an ellipse. Here, the method is applied to estimate the area inside a Molniya satellite orbit. Molniya satellites move in highly elliptical orbits and repeatedly sweep across the same regions of space as they go around Earth. The orbit is modeled as an ellipse in standard form, and...

integration of rational functions using partial fractions

integration-of-rational-functions-using-partial-fractions

Splitting Rational Integrals Into Simpler Parts

Rational functions are ratios of two polynomials, and their integrals can be simplified by breaking them into smaller parts. A rational function is proper when the degree of the numerator is less than the degree of the denominator. When that is true, partial fraction decomposition rewrites the expression as a sum of simpler rational terms. To decompose a proper rational function, unknown constants are placed into the simpler terms. The denominator is then multiplied through to remove the...

rationalizing substitutions

Integrating Radicals with Rational Substitution

Integrating radicals can be easier when a rationalizing substitution turns the expression into a rational form. This method is useful when standard techniques do not work well because the integrand contains a radical, such as a cube root. The transcript uses a rod as a real-world example. Its linear mass density depends on a constant linear density, a characteristic length, and the distance from the left end of the rod. Finding the total mass means integrating that density over the length of...

approximate integration

Riemann Sum Area Estimates

Riemann sums estimate the area under a curve when an exact definite integral is hard to find. This often happens when a function has no known antiderivative or cannot be written in a closed form. It also happens when the function comes from measured data instead of a formula. The method works by splitting the interval of integration into equal parts. Each subinterval gets a rectangle, and the rectangle height comes from the function value at one chosen point in that part. Adding the rectangle...

trapezoidal rule

Estimating Distance from Velocity Data

Estimating distance from velocity data is a common use of the trapezoidal rule. In physics and engineering, velocity is often recorded at separate time points instead of as one continuous equation. Numerical integration gives a way to approximate the total displacement from those measurements. The method divides the full time interval into equal parts. For each part, the velocities at the two endpoints are joined with a straight line on a velocity-time graph. That line creates a trapezoid.

simpson's rule i

Estimating Area with Simpson’s Rule

Simpson’s Rule is a numerical integration method for estimating the value of a definite integral. It is useful when an exact antiderivative is hard or impossible to find. The method works by fitting a unique parabola through three equally spaced points on a curve and then integrating that quadratic over the interval. This approach can give a very accurate area estimate from only a few sampled values. It works well for many smoothly varying functions. That makes it a practical tool when a curve...

simpson's rule ii

Simpson's Rule for Curved Sheet Area

Simpson's Rule helps estimate the area under a smooth curve, such as the curved profile of metal roofing sheets. In warehouse roofing, corrugated or curved sheets are used because they add strength, improve water drainage, and support better ventilation. To plan materials and design more accurately, engineers often need the area of these curved surfaces. The sheet shape can often be treated like a parabolic curve because it changes smoothly along its length. That makes numerical integration a...

improper integrals: infinite intervals

Improper integrals with infinite intervals appear when one limit of integration goes to positive or negative infinity. In that case, the area under the curve is unbounded, so ordinary definite integral methods do not apply directly. The integral is then defined by a limit, which helps show whether the total area is finite even when the interval is infinite. This idea is useful in exponential decay models. One example is the total integrated intensity of light moving through a uniform medium,...

improper integrals: discontinuous integrands

improper-integrals-discontinuous-integrands

Improper Integrals at an Endpoint

Improper integrals can appear when a definite integral has a discontinuity at one endpoint. The integrand may be undefined or become infinite there. In that case, the area under the curve is unbounded near the boundary. This behavior is often linked to a vertical asymptote at the edge of the interval. To evaluate this kind of integral, the endpoint is replaced with a variable. The integral is then written over an interval where the function is defined. After that, a limit is taken as the...

key terms and concepts

Learn how improper integrals on infinite intervals use limits, and see how exponential decay in fog can still produce a finite total intensity.

Improper Integrals and Exponential Decay | JoVE K12

The JoVE Core Calculus collection provides a comprehensive visual guide to the entire calculus sequence, seamlessly connecting mathematical theory to real-world applications in physics and economics. Through dynamic animated lessons, the course transitions smoothly from foundational single-variable concepts to advanced multivariable topics. Students will explore everything from standard differentiation and integration to spatial vectors, multi-dimensional derivatives, and complex vector fields. By breaking down abstract coordinate systems, infinite series, and spatial motion, this structured approach builds the robust analytical skills and advanced mastery essential for STEM success.

calculus

Improper Integrals and Exponential Decay

Description

Transcript

Tags