Например, так OO1 = OA + AA1 + A1O1 = -⅓(AB + AC) + AA1 + ⅓(A1B1 + A1C1) 3·OO1 = -⅓(AB+AC+BC+BA+CA+CB) + AA1 + BB1 + CC1 + ⅓(A1B1+A1C1+B1C1+B1A1+C1A1+C1B1) = AA1 + BB1 + CC1.
Например, так
OO1 = OA + AA1 + A1O1 = -⅓(AB + AC) + AA1 + ⅓(A1B1 + A1C1)
3·OO1 = -⅓(AB+AC+BC+BA+CA+CB) + AA1 + BB1 + CC1 + ⅓(A1B1+A1C1+B1C1+B1A1+C1A1+C1B1) =
AA1 + BB1 + CC1.