x^2+y^2-4x-6y=-13
x^2+y^2-4x-6y+13=0
y^2-6y+9+x^2-4x+4=0
(y-3)^2+(x-2)^2=0；
∴x=2,y=3
∴当x=2,y=3时
(x^2-2y^2)/2xy
=(2^2-2*3^2)/2*2*3
=(4-18)/12
=-(7/6).