令a=x^2+y^2
所以a>=0
(a+2)(a-1)-18<=0
a^2+a-20<=0
(a+5)(a-4)<=0
-5<=a<=4
a>=0
0<=a<=4
x^2-2xy+y^2=(x-y)^2>=0
2xy<=x^2+y^2<=4
所以xy<=2