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

  • question_answer
    If \[|\mathbf{a}|+|\mathbf{b}|\,=\,|\mathbf{c}|\] and \[\mathbf{a}+\mathbf{b}=\mathbf{c},\] then the angle between a and b is        

    A)             \[\frac{\pi }{2}\]

    B)             \[\pi \]

    C)             0

    D)             None of these

    Correct Answer: C

    Solution :

                \[\mathbf{a}+\mathbf{b}=\mathbf{c}\Rightarrow \,|\mathbf{a}{{|}^{2}}+|\mathbf{b}{{|}^{2}}+2\mathbf{a}\,.\,\mathbf{b}=\,|\mathbf{c}{{|}^{2}}\]                    and \[|\mathbf{a}|+|\mathbf{b}|\,=\,|\mathbf{c}|\] \[\Rightarrow \,|\mathbf{a}{{|}^{2}}+|\mathbf{b}{{|}^{2}}+2|\mathbf{a}||\mathbf{b}|\,=\,|\mathbf{c}{{|}^{2}}\]                 \[\therefore \,\mathbf{a}\,.\,\mathbf{b}=\,|\mathbf{a}||\mathbf{b}|\Rightarrow \cos \theta =1\] Þ \[\theta =0.\]


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