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Question

If a,b, c are three non-coplanar non-zero vectors and r is any vector, then
(a×b)×(r×c)+(b×c)×(r×a)+(c×a)×(r×b)=

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

The correct option is B 2[abc]r
We have,
(¯¯¯aׯ¯b)×(¯¯¯rׯ¯c)+(¯¯bׯ¯c)×(¯¯¯rׯ¯¯a)+(¯¯cׯ¯¯a)×(¯¯¯rׯ¯b)

=[¯¯¯a¯¯b¯¯c]¯¯¯r[¯¯¯a¯¯b¯¯¯r]¯¯c+[¯¯¯a¯¯b¯¯c]¯¯¯r[¯¯b¯¯c¯¯¯r]¯¯b+[¯¯¯a¯¯b¯¯c]¯¯¯r[¯¯c¯¯¯a¯¯¯r]¯¯¯a
=3[¯¯¯a¯¯b¯¯c]¯¯¯r[¯¯¯a¯¯b¯¯c]¯¯¯r
=2[¯¯¯a¯¯b¯¯c]¯¯¯r

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