用作差法就可以了,具体如下：
y/(x-y)-z/(x-z)=xy-yz-xz+zy/(x-y)(x-z)=x(y-z)/(x-y)(x-z)
讨论,因为y-z>0,x-z>0,x-y>0
若x>0,则y/(x-y)-z/(x-z)>0,y/(x-y)>z/(x-z)
若x