x:y:z=3:4:5=6,求x-y+z分之x+y-z的值
3y=4x,y=4x/3,
3z=5x,z=5x/3
x-y+z分之x+y-z=（x-4x/3+5x/3）/（x+4x/3-5x/3）=（4x/3）/（2x/3）=2