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Question

Let p,q, r be three mutually perpendicular vectors of the same magnitude. lf a vector x satisfies the equation p×{(xq)×p}+q×{(xr)×q}+r×{(xp)×r}is given by

A
12(p+q2r)
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B
12(p+q+r)
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C
13(p+q2r)
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D
13(2p+qr)
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Solution

The correct option is B 12(p+q+r)
To make the problem easier, let ¯¯¯p=^i,¯¯¯q=^j,¯¯¯r=^k.
LHS becomes (¯¯¯x^j)^i.(¯¯¯x^j)^i+(¯¯¯x^k)^j.(¯¯¯x^k)^j+(¯¯¯x^i)^k.(¯¯¯x^i)^k
0=3(a^i+b^j+c^k)^i^j^ka^ib^jc^k
^i+^j+^k=2a^i+2b^j+2c^k
Comparing co-efficients, we get a=b=c=12
Thus, ¯¯¯x=12(¯¯¯p+¯¯¯q+¯¯¯r)

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