x的二次方+y的二次方-4x+6y+13=0
(x^2-4x+4)+(y^2+6y+9)=0
(x-2)^2+(y+3)^2=0
因为(x-2)^2>=0 (y+3)^2>=0
所以(x-2)^2=(y+3)^2=0
x-2=y+3=0
x=2,y=-3
x+y=2-3=-1