lim[∫(0,x)(x-t)f(t)dt]/∫(0,x)f(x-t)dt=lim[x∫(0,x)f(t)dt-∫(0,x)tf(t)dt]/∫(x,0)f(u)d(-u)=lim[x∫(0,x)f(t)dt-∫(0,x)tf(t)dt]/∫(0,x)f(u)du=(罗比达法则)lim[∫(0,x)f(t)dt+xf(x)-xf(x)]/f(x)=lim∫(0,x)f(t)dt/f(x)=limxf(c)/f(x)=0(c属于(0,x))