x^2+y^2-4x-6y+13=0
(x^2-4x+4)+(x^2-6y+9)=0
(x-2)^2+(y-3)^2=0
所以(x-2)^2=0,(y-3)^2=0
x-2=0,y-3=0
x=2,y=3
xy/(y^2-x^2)
=xy/(y+x)(y-x)
=2*3/5*1
=6/5