对f(x)/x求导,只要证明分子大于0,即f'(x)>f(x)/x,这可利用拉格朗日中值定理,f(x)/x=f'(t),t属于(0,x),由于f''(x)＞0,从而一阶导数单调递增,故f'(x)>f'(t)=f(x)/x