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

  • question_answer
    If the moduli of a and b are equal and angle between them is \[{{120}^{o}}\] and \[\mathbf{a}\,.\,\mathbf{b}=-\,8,\] then | a | is equal to  [RPET 1986]

    A)             ? 5

    B)             ? 4

    C)             4

    D)             5

    Correct Answer: C

    Solution :

               \[\mathbf{a}\,.\mathbf{b}=aa\cos 120{}^\circ \],     \[\left\{ \because \,|\mathbf{a}|\,=\,|\mathbf{b}|\,=a\,(\text{say}) \right\}\]                    \[\Rightarrow -8=-\frac{{{a}^{2}}}{2}\Rightarrow a=4\] (Negative sign does not occur in module).


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