If →a,→b and →c are three non – coplanar vectors, then (→a+→b+→c).((→a+→b)×(→a+→c)) equals
A
→0
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B
[→a→b→c]
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C
2[→a→b→c]
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D
−[→a→b→c]
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Solution
The correct option is D−[→a→b→c] We have,(→a+→b)×(→a+→c)=→a×→c+→b×→a+→b×→c =−→a×b+→b×→c−→c×→a∴(→a+→b+→c).{(→a+→b)×(→a+→c)} =(→a+→b+→c).(−→a×→b+→b×→c−→c×→a) =[→a→a→b]+[→a→b→c]−[→a→c→a]−[→b→a→b]+[→b→b→c]−[→b→c→a]−[→c→a→b]+[→c→b→c]−[→c→c→a]=[→a→b→c]−[→b→c→a]−[→c→a→b]=−[→a→b→c]