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Question

If a,b,c are three noncoplanar nonzero vectors and r is any vector in space then (a×b)×(r×c)+(b×c×(r×a)+(c×a)×(r×b) is equal to

A
2[abc]r
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B
3[abc]r
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C
[abc]r
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D
none of these
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Solution

The correct option is A 2[abc]r
(a×b)×(r×c)=[abc]r[abr]c ......(1)
(a×c)×(r×a)=[bca]r[bcr]a ......(2)
(c×a)×(r×b)=[cab]r[car]b .......(3)
Adding above three, we have
(a×b)×(r×c)+(b×c)×(r×a)+(c×a)(r×b)
=3[abc]r{[abr]c+[bcr]a+[car]b} ..........(4)
Again (a×b)×(r×c)=[arc]b[brcc]a..........(5)
From (1) and (5),
[abc]r[abr]c+[car]+b[bcr]a
From (4) , the expression
= 3[abc]r[abc]r=2[abcr.

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