一、有关概念如果y是u的函数,而u又是x的函数,即y=f(u),u=g(x),x∈A.那么函数y=f(g(x)),x∈A,叫做f和g的复合函数.其中u叫做中间变量.函数y=f(g(x))是二层复合函数,同样可以定义三层复合函数y=f(g(h(x)))和多层复合函数等.我们主要谈二层复合函数,其中,u=g(x)称为内层函数,y=f(u)称为外层函数. First, if the concept of y is a function of u and u is a function of x, that is, y = f (u), u = g (x), x∈A, then the function y = f x∈A is called a compound function of f and g, where u is called an intermediate variable. The function y = f (g (x)) is a two-layer compound function. ))) And multi-layer complex functions, etc. We mainly talk about two-layer complex functions, where u = g (x) is called the inner function and y = f (u) is the outer function.