1. Let u=g(x) and derive f (u): f' (x) = f' (u) * g' (x); 2. Let u = g (x) and a = p (u) and derive f (a): f' (x) = f' (a) * p' (u) * g' (x);

Let the domain of function y=f(u) be Du, the domain of value be Mu, and the domain of function u=g(x) be D_.

M_∩Du≦_, then any x in m _ ∩ du passes through u; If there is a uniquely determined Y value corresponding to it, the functional relationship between variables X and Y is formed by variable U, which is called a composite function.