[(x^2+y^2)-(x^2-y^2)+2y(x-y)]/4
=(2y^2+2xy-2y^2)/4
=xy/2
2x-y=8 y=2x-8
xy/2=x(2x-8)/2=x(x-4)=x^2-4x
如果已知给的是2xy=8
则xy/2=4/2=2