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

  • question_answer
    If q be the angle between the unit vectors a and b, then \[\mathbf{a}-\sqrt{2}\,\mathbf{b}\] will be a unit vector if \[\theta =\]

    A)             \[\frac{\pi }{6}\]

    B)             \[\frac{\pi }{4}\]

    C)             \[p\mathbf{i}+q\mathbf{j}+r\mathbf{k}\]

    D)             \[\frac{2\pi }{3}\]

    Correct Answer: B

    Solution :

               \[{{(\mathbf{a}-\sqrt{2}\mathbf{b})}^{2}}=1\Rightarrow 1+2-2\sqrt{2}\mathbf{a}\,.\,\mathbf{b}=1\]                                 \[\Rightarrow \mathbf{a}\,.\,\mathbf{b}=\frac{1}{\sqrt{2}}\Rightarrow \cos \theta =\frac{1}{\sqrt{2}}\Rightarrow \theta =\frac{\pi }{4}.\]


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