reg = ImplicitRegion[(2 x/3 + y/3 + z/3)^2 + (2 y/3 + x/3 - z/3)^2 + (2 z/3 + x/3 - y/3)^2 <= 1 &&
(2 x/3 - y/3 - z/3)^2 + (2 y/3 - x/3 - z/3)^2 + (2 z/3 - x/3 - y/3)^2 <= 1 &&
(2 x/3 + y/3 - z/3)^2 + (2 y/3 + x/3 + z/3)^2 + (2 z/3 - x/3 + y/3)^2 <= 1 &&
(2 x/3 - y/3 + z/3)^2 + (2 y/3 - x/3 + z/3)^2 + (2 z/3 + x/3 + y/3)^2 <= 1 && x^2 + y^2 <= 1, {x, y, z}];
RegionPlot3D[(2 x/3 + y/3 + z/3)^2 + (2 y/3 + x/3 - z/3)^2 + (2 z/3 +
x/3 - y/3)^2 <= 1 && (2 x/3 - y/3 - z/3)^2 + (2 y/3 - x/3 - z/3)^2 + (2 z/3 - x/3 - y/3)^2 <= 1 &&
(2 x/3 + y/3 - z/3)^2 + (2 y/3 + x/3 + z/3)^2 + (2 z/3 - x/3 +
y/3)^2 <= 1 && (2 x/3 - y/3 + z/3)^2 + (2 y/3 - x/3 + z/3)^2 + (2 z/3 + x/3 +
y/3)^2 <= 1 && x^2 + y^2 <= 1, {x, -3/2, 3/2}, {y, -3/2, 3/2}, {z, -3/2, 3/2}, PlotPoints -> 50]
It is very probablyprobable that the exact result equals 22/5
. Just for sportive interest, how tocan I prove or disprove it with Mathematica? I don't find the answer here.