8
x+y=(a^1/3+b^1/3)^3
x-y=(a^1/3-b^1/3)^3
(x+y)^2/3+(x-y)^2/3=(a^1/3+b^1/3)^2+(a^1/3-b^1/3)^2=2(a^2/3+b^2/3)=8