答：
f(x,y)=3xy / (x^2+y^2)
f(y/x,1)=3*(y/x)*1 / [(y/x)^2+1^2]
=(3y/x) / [(y^2+x^2)/x^2]
=3xy /(x^2+y^2)
=f(x,y)
x≠0
所以：
f(y/x,1)=3xy /(x^2+y^2),x≠0