x^2-y^2分之1/2x^2+xy-1.5y^2
=1/2x^2+xy-1.5y^2/(x^2-y^2)
=(x^2+2xy-3y^2)/[2(x-y)(x+y)]
=(x-y)(x+3y)/[2(x-y)(x+y)]
=(x+3y)/[2(x+y)]
=1/2+[y/(x+y)]