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