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Question

If a, b, c be three non-coplanar vectors and r be any arbitrary vector, then (a×b)×(r×c)+(b×c)×(r×a)+(c×a)×(r×b)

is equal to

A
0
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B
[abc]r
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C
2[abc]r
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D
3[abc]r
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Solution

The correct option is C 2[abc]r
We have, (a×b)×(r×c)=((a×b).c)r((a×b).r)c
=[abc]r[abr]c
Similarly, (b×c)×(r×a)=[bca]r[bcr]c
and (c×a)×(r×b)=[cab]r[car]b
(a×b)×(r×c)+(b×c)×(r×a)+(c×a)×(r×b)=3[abc]r([bcr]a+[car]b+[abr]c)=3[abc]r[abc]r=2[abc]

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