8ax^2-4ax+1/2a-2ay^2
=8a(x^2-1/2ax+1/4)-2ay^2
=8a(x-1/2)^2-2ay^2
=8a[(x-1/2)^2-1/4y^2]
=8a(x+y/2-1/2)(x-y/2-1/2)