x²y²-4xy+2=1-x²-y²x²y²-4xy+1+x²+y²=0(xy-1)²+(x-y)²=0xy-1=x-y=0x=y=1或x=y=-1
x=1，y=1或x=-1，y=-1（xy-1）2+（x-y）2=0
