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Question

If a is a non-zero vector, then the value of ^i×(a×^i)+^j×(a×^j)+^k×(a×^k) is

A
3a
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B
2a
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C
a
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D
0
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Solution

The correct option is B 2a
Let a=ax^i+ay^j+az^k
^i×(a×^i)=((^i^i)a(^ia)^i)
=(a(a^i)¯i)=aax^i
Similarly,
^j×(a×^j)=aay^j
Similarly,
^k×(a×^k)=aaz^k
Adding all above three cases, we get
=^i×(a×^i)+^j×(a×^j)+^k×(a×^k)=3a(ax^i+ay^j+az^k)
=3aa=2a

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