x²-4x+y²+2y+5=0x²-4x+4+y²+2y+1=0(x²-4x+4)+(y²+2y+1)=0(x-2)²+（y+1)²=0(x-2)²=0 x-2=0 x=2(y-1)²=0 y-1=0 y=1