(xy)²-4xy+2=1-x²-y²
(xy)²+x²+y²-4xy+1=0
(xy)²-2xy+1+(x²+y²-2xy)=0
(xy-1)²+(x-y)²=0
(xy-1)²=-(x-y)² (将出现虚数)
∴ xy-1=0,x-y=0
xy=1,x=y
(xy)²-4xy+2=1-x²-y²
(xy)²+x²+y²-4xy+1=0
(xy)²-2xy+1+(x²+y²-2xy)=0
(xy-1)²+(x-y)²=0
(xy-1)²=-(x-y)² (将出现虚数)
∴ xy-1=0,x-y=0
xy=1,x=y