=(a/2 - b/2)[2/3(a/2+b/2)]+b^2
=2/3(a/2 - b/2)(a/2+b/2)+b^2
=2/3[(a/2)^2-(b/2)^2]+b^2
=(2/3)·(a^2-b^2)/4+b^2
=(a^2-b^2)/6+6b^2/6
=(a^2+5b^2)/6