x^2+根号2y=根号3,
y^2+根号2x=根号3;两式相减；（x-y)(x+y-根号2)=0;
x不等于y,x+y=根号2;
两式相加：x^2+y^2+根号2(x+y)=2根号3;
(x+y)^2-2xy=2根号3-2,
xy=2-根号3