（1-x2)(1-y2)+4xy=1-y2-x2+x2y2+4xy=2xy-y2-x2+(1+xy)^2=-(x-y)^2+(1+xy)^2=48
(xy+x-y+1)(xy-x+y+1)=(xy+1)^2-(x-y)^2
即48=6(xy-x+y+1)
xy-x+y+1=8