设X/3=Y/5=Z/7=t,则X=3t,Y=5t,Z=7t.所以 (X-Y+Z)/(X+Y-Z)=(3t-5t+7t)/(3t+5t-7t) =5t/t =5
设x=3t,y=5t,z=7t,故最后等于5
