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[→a→b→c]→r
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B
3[→a→b→c]→r
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C
[→a→b→c]→r
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D
none of these
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Solution
The correct option is A2[→a→b→c]→r (→a×→b)×(→r×→c)=[→a→b→c]→r−[→a→b→r]→c ......(1) (→a×→c)×(→r×→a)=[→b→c→a]→r−[→b→c→r]→a ......(2) (→c×→a)×(→r×→b)=[→c→a→b]→r−[→c→a→r]→b .......(3) Adding above three, we have (→a×→b)×(→r×→c)+(→b×→c)×(→r×→a)+(→c×a)(→r×→b) =3[→a→b→c]→r−{[→a→b→r]→c+[→b→c→r]→a+[→c→a→r]→b} ..........(4) Again (→a×→b)×(→r×→c)=[→a→r→c]→b−[→b→r→cc]→a..........(5) From (1) and (5), [→a→b→c]→r−[→a→b→r]→c+[→c→a→r]+→b[→b→c→r]→a ∴ From (4) , the expression = 3[→a→b→c]→r−[→a→b→c]→r=2[→a→b→c→r.