x²+4x+y²-6y+13=0
(x+2)^2+(y-3)^2=0
由于(x+2)^2>=0 (y-3)^2>=0
所以x+2=y-3=0
x=-2 y=3
x-2y/x²+y²=(-2-6)/(4+9)=-8/13