x^2+y^2=1
-1<=x<=1 -1<=y<=1
x^2+y^2>=2xy
1>=2xy
(xy)^2<=1/4
(1-xy)(1+xy)=1-(xy)^2>=1-1/4=3/4
又因为(xy)^2>=0 所以1-(xy)^2<=1
综合最大值是1,最小值是3/4