x²+xy-2y²=7x²-y²+xy-y²=7(x+y)(x-y)+y(x-y)=7(x-y)(x+2y)=7又x、y为整数，所以(x-y)>0;x+2y>0;所以 x-y=1,x+2y=7;或者 x-y=7;x+2y=1;若要实现xy为整数，只能是第一种情况，即x=3,y=2
