Given
→a,→b and
→c are three non- coplanar vectors
To prove: [→a+→b+→c→a+→b→a+→c]=−[→a→b→c]
Sol: [→a+→b+→c→a+→b→a+→c]
=(→a+→b+→c).[(→a+→b)×(→a+→c)]
=(→a+→b+→c).(→a×→a+→a×→c+→b×→a+→b×→c)
=(→a+→b+→c).(→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×→c)+→c.(→b×→a)
=[→a→a→c]+[→a→b→a]+[→a→b→c]+[→b→a→c]+[→b→b→a]+[→b→b→c]+[→c→a→c]+[→c→b→c]+[→c→b→a]
We know that if there are two same vectors in a box product, then the product equal to zero.
⟹[→a→a→c]=0
∴0+0+[→a→b→c]+[→b→a→c]+0+0+0+[→c→b→a]+0
[→a→b→c]+[→b→a→c]+[→c→b→a]
=[→a→b→c]−[→a→b→c]−[→a→b→c]
=−[→a→b→c]
Hence proved.