X²-8X+Y²-6Y+25=0
x^2-8x+16+y^2-6y+9=0,
(x^2-8x+16)+(y^2-6y+9)=0
(x-4)^2+(y-3)^2=0
所以x-4=0,y-3=0,
解得x=4,y=3
所以x+y=7