Théorème des valeurs intermédiaires

The Intermediate Value Theorem states that if a function f is continuous on a closed interval [a, b], and N is any value between f(a) and f(b), then there exists at least one point c in the open interval (a, b) where f(c) = N. This guarantees that continuous functions attain all intermediate values between their endpoints.

If a continuous function has opposite signs at two endpoints—one negative and one positive—the Intermediate Value Theorem guarantees the function crosses zero somewhere between them. This allows you to narrow down intervals and approximate solutions even when direct algebraic methods are complex or intractable.

Graphically, the theorem means a continuous curve connecting two points will intersect every horizontal line between the function values at those points. Since continuous functions have no jumps or holes, if a horizontal line y = N lies between f(a) and f(b), the curve must cross that line at least once on the interval.

Continuity ensures a function has no jumps, breaks, or holes over an interval. Without continuity, a function could skip over intermediate values entirely. The theorem relies on this unbroken behavior to guarantee that every value between f(a) and f(b) is actually attained by the function somewhere in the interval.

No, the Intermediate Value Theorem guarantees the existence of at least one solution but does not guarantee uniqueness. Multiple values of c may satisfy f(c) = N within the interval. The theorem only confirms that solutions exist, not how many exist or where exactly they occur.

A roller coaster's path can be modeled as a continuous function. If the function is negative at one point and positive at another relative to a reference level, the Intermediate Value Theorem guarantees the roller coaster crosses that reference level at least once. This applies to any continuous physical process changing between two states.

Consider finding where ln(x) = 1 on the interval [2, 3]. At x = 2, ln(2) ≈ 0.693 (negative relative to 1), and at x = 3, ln(3) ≈ 1.099 (positive relative to 1). Since the logarithmic function is continuous and 0 lies between these values, the theorem guarantees a solution exists near x ≈ 2.718 within the interval.