DefinitionThe multi-variable chain rule is a generalization of the chain rule in single-variable calculus to functions of several variables. It says that if there is a function f of several variables u, v, and if each variable in u, v depends on another set of variables x,y, then you can compute the partial derivatives of f with respect to x,y in terms of the partial derivatives of u,v wrt x,y. The chain rule of multivariate functions is a crucial tool and frequently used when finding the derivative of composite functions in multivariable calculus, in physics, engineering, and optimization problems with many parameters. Example: In economics, it is common to use the production function f(x,y) to describe the output of a firm as a function of the labor input x and capital input y, and the chain rule to compute the marginal product of labor and capital (which gives the impact of changing labor and capital inputs on output slightly )