Name: using continuity to find roots
Uploaded: 2025-11-26T05:46:04.000Z
Duration: 1 min 25 s
Channel: JoVE

Le théorème des valeurs intermédiaires est un principe fondamental du calcul qui s’applique aux fonctions continues.Le théorème stipule que lorsqu’une fonction f est continue sur l’intervalle fermé [a, b], et que N est une valeur comprise entre f(a) et f(b), alors il y aura un point c à l’intérieur de l’intervalle ouvert (a, b) tel que f(c) = N.Graphiquement, le théorème implique qu’une courbe continue reliant deux points, A et B, coupera chaque droite horizontale entre les valeurs de la fonction en ces points.Une application pratique du théorème des valeurs intermédiaires consiste à déterminer où une fonction est égale à zéro sur un intervalle. Si les valeurs de la fonction aux extrémités ont des signes opposés, elle doit être égale à zéro. Cela permet d’approximer une solution en réduisant l’intervalle.Prenons l’exemple de la trajectoire d’une montagne russe, modélisée par un polynôme cubique sur un intervalle par rapport à un niveau de référence.Si la valeur de la fonction est négative en un point et positive en un autre, et que la fonction est continue, le théorème garantit qu’elle est égale à zéro en un point.Cela signifie que les montagnes russes franchiront le niveau de référence au moins une fois dans l’intervalle.

théorème des valeurs intermédiaires

Le th&#233;or&#232;me des valeurs interm&#233;diaires est un r&#233;sultat fondamental de l&#8217;analyse qui assure l&#8217;existence de solutions sur certains intervalles pour des fonctions continues. Formellement, le th&#233;or&#232;me des valeurs interm&#233;diaires &#233;nonce que si une fonction f est continue sur l&#8217;intervalle ferm&#233; [a, b], et si N est une valeur quelconque comprise entre f(a) et f(b), alors il existe au moins un c &#8712; (a, b) tel que f(c) = N. Ce th&#233;or&#232;me est d&#233;terminant pour &#233;tablir l&#8217;existence de z&#233;ros et pour analyser le comportement des fonctions continues sur des intervalles.
Graphiquement, une fonction continue ne pr&#233;sente ni saut ni trou. Si la valeur N est comprise entre f(a) et f(b), alors la droite horizontale y = N intersecte au moins une fois la courbe sur l&#8217;intervalle (a, b). Cette intersection garantit que f(c) = N pour un certain c. Le th&#233;or&#232;me ne garantit pas l&#8217;unicit&#233; ; plusieurs valeurs de c peuvent satisfaire la condition.
Consid&#233;rons la fonction logarithmique
40108_Equation_1.svg
L&#8217;objectif est de d&#233;terminer une valeur x pour laquelle f(x) = 0, ou, de mani&#232;re &#233;quivalente, ln(x) = 1. L&#8217;&#233;valuation aux bornes de l&#8217;intervalle [2, 3] donne :
40108_Equation_2.svg
Puisque la fonction est continue et que la valeur 0 est comprise entre f(2) et f(3), le th&#233;or&#232;me des valeurs interm&#233;diaires garantit l&#8217;existence d&#8217;une solution dans l&#8217;intervalle (2, 3). La solution, x &#8776; 2,718, se situe dans cet intervalle et satisfait l&#8217;&#233;quation.
Cet exemple illustre la puissance du th&#233;or&#232;me des valeurs interm&#233;diaires pour confirmer la pr&#233;sence de racines lorsqu&#8217;une r&#233;solution alg&#233;brique directe serait complexe ou peu commode.

Théorème des valeurs intermédiaires

Le th&#233;or&#232;me des valeurs interm&#233;diaires est un r&#233;sultat fondamental de l&#8217;analyse qui assure l&#8217;existence de solutions sur ...

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The Intermediate Value Theorem

using continuity to find roots

The Intermediate Value Theorem uses continuity to show when a function must pass through a value between two endpoints. If a function is continuous on a closed interval [a, b], and N is any number between f(a) and f(b), then there is at least one c in (a, b) where f(c) = N. This result helps students prove that a solution exists even when the exact answer is hard to find.

A continuous function has no jumps or holes in its graph. If a horizontal line y = N falls between the endpoint values, the graph must cross that line somewhere in the interval. The theorem does not say that there is only one crossing. More than one value of c can satisfy the same equation.

The theorem is especially useful for finding roots, or x-values where f(x) = 0. For example, for the logarithmic function, the goal is to solve ln(x) = 1. When the interval [2, 3] is checked, the endpoint values show that 0 lies between f(2) and f(3). Because the function is continuous, the Intermediate Value Theorem guarantees a solution in (2, 3), and the exact value is about 2.718. This example shows how the theorem can confirm a root even when direct algebra is difficult.

Using Continuity to Find Roots

tangent to a curve

Secant Lines Approaching a Tangent

Secant lines and tangent lines help describe how a curve changes at a chosen point. The curve shown here comes from a function where each output is the square of the input. It bends upward and becomes steeper as you move farther from the center. To study the curve near one position, a second point is chosen slightly ahead on the graph. A straight line through the two points is called a secant line. It crosses the curve and shows the average rise or fall between those points. Because it spans a...

velocity of an object

Finding Instantaneous Velocity from Motion

Finding instantaneous velocity from motion starts with a simple case: a vehicle moving along a straight, level path. Its position can be tracked at any time. The first step is to measure how far the vehicle travels over a fixed time period. That measurement is average velocity. It is found by dividing the total change in position by the time interval. Average velocity gives a broad picture of how quickly the vehicle moves between two times. It does not show how the motion changes at one exact...

introduction to limits

Limits in Graphs and Real-World Change

Limits describe the value a function approaches as its input gets closer to a point. They are useful even when the function is undefined at that exact value. This makes limits a key idea in calculus and an important step toward understanding continuity, derivatives, and integrals. In math, a function f(x) has a limit L at x = a when its output approaches L as x gets very close to a. This idea is written with limit notation, and it does not require f(a) to be defined. The notation shows the...

types of limits i

One-Sided Limits and Limit Failure

Limits describe how a function behaves as its input gets close to a target value. They are useful when the function is undefined at that point. Instead of focusing on the exact value, limits show the trend near a critical point and can reveal a discontinuity. One-sided limits look at approach from a single direction. A left-hand limit checks values coming from the left, while a right-hand limit checks values coming from the right. If a function changes differently from each side, the two...

types of limits ii

Vertical Asymptotes and Infinite Limits

Vertical asymptotes and infinite limits describe how a curve behaves near a point where its height grows without bound. As the position on the horizontal axis gets closer to that point, the graph does not settle at a single value. Instead, the values rise or fall more and more sharply. At the point itself, no defined value exists. The curve may climb steeply on one side and drop just as sharply on the other. This creates a strong contrast near the same x-value. On the graph, this behavior...

limits with oscillating discontinuities

Oscillating Limits Near Discontinuities

Oscillating discontinuities happen when a function keeps changing value as the input gets close to a point. The output does not settle down to one number. Because of that, the limit at that point does not exist. This kind of discontinuity is different from a jump discontinuity and an infinite discontinuity. In a jump discontinuity, the function shifts suddenly between two values. In an infinite discontinuity, the function grows without bound. A well-known example comes from the function...

limit laws i

Limit Laws for Combining Functions

Limit laws help students find limits of combined functions. These rules show how a function behaves as its input approaches a specific value. They are useful when the limit of each separate function exists. The Sum and Difference Laws say that the limit of a sum or difference equals the sum or difference of the individual limits. The Product Law says that the limit of a product equals the product of the separate limits. These rules make many limit problems easier to solve. A car rental cost...

limit laws ii

Quotient, Power, and Root Limits

Limit laws help students break down complicated expressions in calculus. They make it easier to find a limit by working with smaller parts of a function. In this lesson, the focus is on quotients, powers, and roots. The Quotient Law applies to a division of two functions. It says the limit of the quotient can be found by dividing the limits of the top and bottom functions, as long as the limit of the denominator exists and is not zero. This rule is useful when a limit problem contains a...

evaluating limits by direct substitution

Using Continuity to Find Limits

Using continuity to find limits is a simple way to evaluate a function near a specific input value. This method works for algebraic expressions that have no breaks, holes, or jumps near the point of interest. When a function is continuous at that point, the limit can often be found by direct substitution. Direct substitution means replacing the variable with the value it approaches. The result gives the output value as the input nears that point. In a continuous function, the value near the...

the squeeze theorem

Using Bounds to Find a Limit

Using bounds to find a limit is the main idea behind the Squeeze Theorem. It helps when a function changes in a way that makes direct limit evaluation difficult. Some functions may oscillate quickly or follow irregular patterns near a specific input value. The theorem works by placing the function between two other functions near that point. The function must stay greater than or equal to a lower bound and less than or equal to an upper bound. If both outer functions approach the same limit as...

the precise definition of a limit

Epsilon-Delta Logic for Limits

Epsilon-delta logic gives a precise way to describe a limit in calculus. It shows how a function behaves near a chosen point without using vague words like "getting close." This makes limit analysis clear and logically consistent. The formal definition says that the limit of f(x) as x approaches a is L. In symbols, for every epsilon greater than 0, there exists a delta greater than 0 such that the function values stay within epsilon of L whenever x stays within delta of a. In other words, no...

continuity of a function

Types of Function Discontinuities

A continuous function has no breaks, holes, or jumps at a point. For a function to be continuous at a point a, three conditions must be met. The function must be defined at a, the limit as x approaches a must exist, and that limit must equal the function’s value. When any of these conditions fail, the function is discontinuous. One common case is a removable discontinuity. In this case, the two-sided limit exists, but the function is either undefined at the point or given the wrong value. This...

properties of continuous functions

How Continuous Functions Stay Smooth

Continuous functions stay smooth when they are added, subtracted, multiplied by a constant, or multiplied together. If f and g are continuous at a point a, then f+g, f-g, cf, and fg are also continuous at a. This makes it possible to build more complex functions from simpler continuous parts without breaking continuity. Polynomials are continuous for every real number. A polynomial is an expression made from sums of powers of x with constant coefficients. So polynomial graphs are continuous...

the intermediate value theorem

The Intermediate Value Theorem uses continuity to show when a function must pass through a value between two endpoints. If a function is continuous on a closed interval [a, b], and N is any number between f(a) and f(b), then there is at least one c in (a, b) where f(c) = N. This result helps students prove that a solution exists even when the exact answer is hard to find. A continuous function has no jumps or holes in its graph. If a horizontal line y = N falls between the endpoint values, the...

limits at infinity

Horizontal Asymptotes in Function Graphs

Horizontal asymptotes describe how a function behaves as the input grows very large. In this topic, a decreasing function gets smaller and smaller as the input increases. Its output moves closer and closer to one fixed value. The function does not reach that value, but it approaches it without limit. On a graph, this long-term behavior appears as a curve that flattens out. The graph seems to level off near a horizontal line. That line is the horizontal asymptote, which the curve approaches but...

key terms and concepts

Learn how the Intermediate Value Theorem uses continuity to prove a value or root exists between two endpoints, with a logarithm example on the interval [2, 3].

Using Continuity to Find Roots | JoVE K12

The JoVE Math Fundamentals collection is designed to build a deep and comprehensive understanding of essential mathematics. Covering the foundations of algebra, geometry, trigonometry, and analytic reasoning, this course establishes a strong base for future studies. The collection connects core mathematical concepts with real-world practice, equipping learners to solve complex problems, master problem-solving skills, and apply abstract ideas.

math fundamentals

Using Continuity to Find Roots

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