记u=x+y, v=x-y则解得：x=(u+v)/2, y=(u-v)/2代入等式得：f(u,v)=(u+v)^2/4-(u+v)/2*(u-v)/2即f(u,v)=(u^2+v^2+2uv)/4-(u^2-v^2)/4=(v^2+uv)/2即f(x,y)=(y^2+xy)/2
