因 f(x^2-y^2,x-y)=x+y+x^2-xy =(x^2-y^2)/(x-y) +[(x^2-y^2)+(x-y)^2 ] /2
所以f(x,y)=x/y+(x+y^2)/2,
fy(x,y)= -x / (y^2)+y