JEE Main & Advanced Mathematics Vector Algebra Question Bank Scaler or Dot product of two vectors and its application

  • question_answer
    If \[\mathbf{p}=\mathbf{i}-2\mathbf{j}+3\mathbf{k}\] and \[\mathbf{q}=3\mathbf{i}+\mathbf{j}+2\mathbf{k},\] then a vector along r which is linear combination of p and q and also perpendicular to q is                           [MNR 1986]

    A)             \[\mathbf{i}+5\mathbf{j}-4\mathbf{k}\]

    B)             \[\mathbf{i}-5\mathbf{j}+4\mathbf{k}\]

    C)             \[-\frac{1}{2}\,(\mathbf{i}+5\mathbf{j}-4\mathbf{k})\]

    D)             None of these

    Correct Answer: C

    Solution :

               \[\mathbf{r}=\mathbf{p}+\lambda \,\mathbf{q}\Rightarrow \mathbf{r}\,.\,\mathbf{q}=\mathbf{p}\,.\,\mathbf{q}+\lambda \,\mathbf{q}\,.\,\mathbf{q}\]                    \[\Rightarrow 0=7+14\lambda \,\,\Rightarrow \lambda \,\,=-\frac{1}{2}\]                                 Therefore, \[\mathbf{r}=-\frac{1}{2}(\mathbf{i}+5\mathbf{j}-4\mathbf{k}).\]


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