X=根号2+1分之根号2-1,Y=根号2-1分之根号2+1
X^2=（根号2-1）^2=3-2根号2
XY=1
Y^2=（根号2+1）^2=3+2根号2
X^2-XY＋ Y^2=3-2根号2-1+3+2根号2=5