设x/2=y/3=z/4=a,则x=2a,y=3a,z=4a
xy+yz+xz=2a*3a+3a*4a+4a*2a=26a*a
xx+yy+zz=2a*2a+3a*3a+4a*4a=29a*a
则结果为26/29