设a/b=x，c/d=y，则y>x>0,a=bx,c=dy所以(a+c)/(b+d)=(bx+dy)/(b+d)<(by+dy)/(b+d)=y=c/d,(a+c)/(b+d)=(bx+dy)/(b+d)>(bx+dx)/(b+d)=x=a/b.
