设x/2=y/3=z/4=a
则:x=2a;y=3a;z=4a
代入得：
(xy+yz+zx)/(x^2+y^2+z^2)
=(6a^2+12a^2+8a^2)/(4a^2+9a^2+16a^2)
=26a^2/29a^2
=26/29