If →a,→b and →c are three non-coplanar vectors, then (→a+→b+→c)⋅[(→a+→b)×(→a+→c)] equals
−[→a→b→c]
(→a+→b+→c).[(→a+→b)×(→a+→c)]=(→a+→b+→c).[→a×→a+→a×→c+→b×→a+→b×→c]={→a.(→a×→c)+→a.(→b×→a)+→a.(→b×→c)}+{→b.(→a×→c)+→b.(→b×→a)+→b.(→b×→c)}+{→c.(→a×→c)+→c.(→b×→a)+→c.(→b×→c)}⇒[→a→b→c]+[→b→a→c]+[→c→b→a]⇒−[→a→b→c]