La règle de la chaîne

The Chain Rule states that the derivative of a composite function equals the derivative of the outer function multiplied by the derivative of the inner function. It allows you to find instantaneous rates of change for functions composed of multiple stages. This rule is essential for differentiating complex functions where one variable depends on another through an intermediate variable.

A three-gear system demonstrates the Chain Rule through sequential dependencies. The first gear drives the second, which drives the third. Each gear's rotational speed depends on the preceding gear, creating a composite function relationship. The overall rate of change between the first and third gears equals the product of individual rates, directly reflecting the Chain Rule's structure.

A composite function combines two or more functions where the output of one becomes the input of the next. In the gear system, if z depends on x and y depends on z, then y depends on x through z as an intermediate variable. This creates the composite relationship y = y(z(x)), which the Chain Rule directly addresses.

The intermediate variable bridges the relationship between outer and inner functions. By introducing delta z as an intermediate change, you can express the ratio Δy/Δx as a product of two separate ratios: Δy/Δz and Δz/Δx. As changes approach zero, these ratios become derivatives, allowing the Chain Rule formula to emerge naturally from the limit definition.

To find the derivative of y = f(g(x)), multiply the derivative of the outer function f evaluated at g(x) by the derivative of the inner function g(x). Mathematically, dy/dx = (dy/dz)(dz/dx), where z is the intermediate variable. This product of derivatives gives the instantaneous rate of change of the composite function.

As the change in the independent variable approaches zero, the intermediate change also approaches zero. This simultaneous approach to zero allows each ratio in the product to be interpreted as a derivative. The limiting process transforms the ratio of finite changes into the product of instantaneous rates, which is the essence of the Chain Rule.

The Chain Rule applies recursively to functions with multiple nested layers. For a function like y = f(g(h(x))), you multiply the derivatives of each layer: dy/dx = (dy/dg)(dg/dh)(dh/dx). Each intermediate variable creates a new stage in the product, allowing you to handle arbitrarily complex composite functions systematically.