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Question

If a,b and c are three non-coplanar vectors, prove that
[a+b+ca+ba+c]=[a b c].

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Solution

Given a,b and c are three non- coplanar vectors
To prove: [a+b+ca+ba+c]=[abc]
Sol: [a+b+ca+ba+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)
=[aac]+[aba]+[abc]+[bac]+[bba]+[bbc]+[cac]+[cbc]+[cba]
We know that if there are two same vectors in a box product, then the product equal to zero.
[aac]=0
0+0+[abc]+[bac]+0+0+0+[cba]+0
[abc]+[bac]+[cba]
=[abc][abc][abc]
=[abc]
Hence proved.

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