OK, you are right, the answer should be 2\sqrt{2}/\sqrt{3} π Let the cone angle be θ, then the volume is expressed as (π l^3 sin^2 θ cos θ)/3 Set t=cos θ, then the volume = (π l^3 / 3)(t-t^3), where t∈[-1,1] Taking derivative gives t=1/\sqrt{3} should be the maximum point, which implies the desired result.