1.

Vectors `veca, vecb, vecc` are three unit vectors and `vecc` is equally inclined to both `veca` and `vecb`. Let `veca xx (vecb xx vecc) + vecb xx (vecc xx veca)` `=(4+x^(2))vecb-(4xcos^(2)theta)veca`, then `veca` and `vecb` are non-collinear vectors, `x gt 0`A. `x=2`B. `theta=0^(@)`C. `theta=x`D. `x=4`

Answer» Correct Answer - A::B::C
`|veca|=|vecb|=|vecc|=1`
and `veca.vecc=vecb.vecc`………….(i)
`veca xx (vecb xx vecc) +vecb xx (vecc xx veca)`
`=(vecc. vecc)vecb-(veca.vecb)vecc+(veca.vecb)vecc-(vecb.vecc)veca`
`=(veca.vecc)vecb-(vecb.vecc)veca`
`rArr veca.vecc+x^(2)` and `vecb.vecc=4xcos^(2)theta`. ........(ii)
From (i) and (ii), `4+x^(2)=4xcos^(2)theta`
`rArr 4+x^(2)=4x-4sin^(2)theta`
`rArr (x-2)^(2)=-4xsin^(2)theta`
`rArr x=2` or `sintheta=0`.


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