復(fù)合函數(shù)的概念：一般地,對(duì)于兩個(gè)函數(shù)y=f(u)和u=g(x),如果通過變量u,y可以表示成x的函數(shù),那么稱這個(gè)函數(shù)為函數(shù)y=f(u)和u=g(x)的復(fù)合函數(shù),記做y=f(g(x)).
復(fù)合函數(shù)的導(dǎo)數(shù)：復(fù)合函數(shù)y=f(g(x))的導(dǎo)數(shù)和函數(shù)y=f(u),u=g(x)的導(dǎo)數(shù)間的關(guān)系為
y'=u'*x'
即y對(duì)x的導(dǎo)數(shù)等于y對(duì)u的導(dǎo)數(shù)與u對(duì)x的導(dǎo)數(shù)的乘積.
設(shè)y=√（x^2+1）
u=x^2+1
y=u^(1/2)
y'=(1/2)u^(-1/2) * 2x
y'=x/√(x^2+1)
f（x）=√（x^2+1）-ax
f'(x)=x/√(x^2+1)-a