设x/3=y/2=z/5=t,则x=3t,y=2t,z=5t
(x²-y²-z²)/(xy+yz+zx)
=[(3t)²-(2t)²-(5t)²]/(3t×2t+2t×5t+5t×3t)
=(9t²-4t²-25t²)/(6t²+10t²+15t²)
=(-20t²)/(31t²)
= -20/31