[(1/9(x+y)^3-2(x+y)^2-2/3(x+y)]/[(x+y)/3]
=(x+y)[1/9(x+y)^2-2(x+y)-2/3]/[(x+y)/3]
=[1/9(x+y)^2-2(x+y)-2/3]/[1/3]
=3*[1/9(x+y)^2-2(x+y)-2/3]
=(x+y)^2/3-6(x+y)-2
=x^2/3+2xy/3+y^2/3-6x-6y-2