设f(x+y,x-y)=x^2+xy
设u=x+y,v=x-y
则x=（u+v）/2,y=(u-v)/2
所以f(u,v)=(u+v)^2/4+(u+v)(u-v)/4
所以f(x,y)=(x+y)^2/4+(x+y)(x-y)/4=(x^2+xy)/2