若f(x)在[a,b]上连续,在(a,b)内可导,且f′(x)<0,令F(x)=1x?a∫xaf(t)dt,a<x<b,证明

若f(x)在[a,b]上连续,在(a,b)内可导,且f′(x)<0,令F(x)=1x?a∫xaf(t)dt,a<x<b,证明:F(x)在(a,b)内单调减少.