x/3=y/4=z/5=k
x=3k,y=4k,z=5k
xy+yz+zx/x²+y²+z²
=(12k²+20k²+15k²)/(9k²+16k²+25k²)
=47/50这个k从哪来的啊设x/3=y/4=z/5=k