The chain rule is a fundamental concept in calculus used to find the derivative of a composite function.

If you have a function f(x) and another function g(u) where u is a function of x, then the chain rule is expressed as: (d/dx) g(f(x)) = g'(f(x)) * f'(x)

In simpler terms, to find the derivative of a composite function, you take the derivative of the outer function evaluated at the inner function, and then multiply it by the derivative of the inner function.

For example: If h(x) = sin(2x), you can consider g(u) = sin(u) where u = 2x. Applying the chain rule: (d/dx) sin(2x) = cos(2x) * 2 = 2cos(2x)

The chain rule is a powerful tool for finding the derivatives of composite functions, which are functions within functions.