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Question

If ¯¯¯a,¯¯b,¯¯c are three non-coplanar vectors, ¯¯¯p,¯¯¯q,¯¯¯r are non-zero vectors such that p=b×c[abc],q=c×a[abc],r=a×b[abc], then the value of the expression (a+b+c).(p+q+r) is

A
0
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B
4
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C
2
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D
3
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Solution

The correct option is C 3
p=b×c[abc],q=c×a[abc],r=a×b[abc]
(a+b+c).p=a.(b×c)[abc]+b.(b×c)[abc]+c.(b×c)[abc]
=[abc][abc]+0+0=1
q.(a+b+c)=a.(c×a)[abc]+b.(c×a)[abc]+c.(c×a)[abc]
=0+[bca][abc]+0=1
[abc]=[bca]=[cab]
=0+[bca][abc]+0=1[abc]=[bca]=[cab]
q.(a+b+c)=1
r.(a+b+c)=a.(a×b)[abc]+b.(a×b)[abc]+c.(a×b)[abc]
=0+0+[cab][abc]=1
(a+b+c).(p+q+r)=p.(a+b+c)+q.(a+b+c)+r.(a+b+c)
1+1+1=3

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