设F(a)=∫(0,a)f(x)dx/a
则:
F‘(a)=[af(a)-∫(0,a)f(x)dx]/a^2
=∫(0,a)(f(a)-f(x))dx/a^2
因为x《a,f(x)在[0,1]是单调递减,故f(a)-f(x)