а) B1C1 + AB + BB1 + B1A1 = C1B1 + CA + AB + A1B1 = -AB + AB + AB + AB = \vec{0}
б) DC - BB1 - AC = DC - BC - CA = DD1
а) B1C1 + AB + BB1 + B1A1 = C1B1 + CA + AB + A1B1 = -AB + AB + AB + AB = \vec{0}
б) DC - BB1 - AC = DC - BC - CA = DD1