With respect to a rectangular cartesian coordinate system, three vectors are expressed as
$\vec a = 4\hat i - \hat j$, $\vec b = - 3\hat i + 2\hat j$ and $\vec c = - \hat k$ 
where $\hat i,\,\hat j,\,\hat k$ are unit vectors, along the $X, Y $ and $Z-$axis respectively. The unit vectors $\hat r$ along the direction of sum of these vector is

  • A
    $\hat r = \frac{1}{{\sqrt 3 }}(\hat i + \hat j - \hat k)$
  • B
    $\hat r = \frac{1}{{\sqrt 2 }}(\hat i + \hat j - \hat k)$
  • C
    $\hat r = \frac{1}{3}(\hat i - \hat j + \hat k)$
  • D
    $\hat r = \frac{1}{{\sqrt 2 }}(\hat i + \hat j + \hat k)$

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