Sistema cartesiano, {(x,y,z)∣0≤x≤a,0≤y≤a,0≤z≤a}\{(x, y, z) | 0\le x\le a, 0\le y\le a, 0\le z\le a\}{(x,y,z)∣0≤x≤a,0≤y≤a,0≤z≤a}