(x+y-2xy)(x+y-2)+(xy-1)^2
=(x+y)^2-2(xy+1)+4xy+(xy-1)^2
=(x+y)^2-2(xy+1)++(xy+1)^2
=(x+y+xy+1)^2
(x+1)^2(y+1)^2第一步的2（xy+1）+4xy是怎么得出来的？x+y-2xy)(x+y-2)+(xy-1)^2=(x+y)^2-2(xy+1)(x+y)+4xy+(xy-1)^2=(x+y)^2-2(xy+1)(x+y)+(xy+1)^2=(x+y+xy+1)^2=(x+1)^2(y+1)^2