x=根号6+2=根号2（根号3+根号2）y=根号3-根号2 则 xy=根号2(x+1/y)+(y+1/x)=(xy+1)(xy+1)/(xy)=(1+根号2)^2/根号2=（3+2根号2）/根号2=3/2 *根号2+2
