Manipal Engineering Manipal Engineering Solved Paper-2010

  • question_answer
    If\[V\]is the volume of the parallelepiped having three coterminus edges as\[\overrightarrow{\mathbf{a}},\,\,\overrightarrow{\mathbf{b}}\]and\[\overrightarrow{\mathbf{c}}\], then the volume of the parallelepiped having three coterminus edge as \[\overrightarrow{\alpha }=(\overrightarrow{\mathbf{a}}\cdot \overrightarrow{\mathbf{a}})\overrightarrow{\mathbf{a}}+(\overrightarrow{\mathbf{a}}\cdot \overrightarrow{\mathbf{b}})\overrightarrow{\mathbf{b}}+(\overrightarrow{\mathbf{a}}\cdot \overrightarrow{\mathbf{c}})\overrightarrow{\mathbf{c}}\], \[\overrightarrow{\beta }=(\overrightarrow{\mathbf{a}}\cdot \overrightarrow{\mathbf{b}})\overrightarrow{\mathbf{a}}+(\overrightarrow{\mathbf{b}}\cdot \overrightarrow{\mathbf{b}})\overrightarrow{\mathbf{b}}+(\overrightarrow{\mathbf{b}}\cdot \overrightarrow{\mathbf{c}})\overrightarrow{\mathbf{c}}\] \[\overrightarrow{\gamma }=(\overrightarrow{\mathbf{a}}\cdot \overrightarrow{\mathbf{c}})\overrightarrow{\mathbf{a}}+(\overrightarrow{\mathbf{b}}\cdot \overrightarrow{\mathbf{c}})\overrightarrow{\mathbf{b}}+(\overrightarrow{\mathbf{c}}\cdot \overrightarrow{\mathbf{c}})\overrightarrow{\mathbf{c}}\]

    A) \[{{V}^{3}}\]                                      

    B) \[3V\]

    C) \[{{V}^{2}}\]                      

    D)        \[2V\]

    Correct Answer: A

    Solution :

    We have,                 \[|[\overrightarrow{\mathbf{a}}\overrightarrow{\mathbf{b}}\overrightarrow{\mathbf{c}}]|\,=V\] Let\[{{V}_{1}}\]be the volume of parallelepiped formed by the vectors\[\vec{\alpha },\,\,\vec{\beta }\]and\[\vec{\gamma }\]. Then,                 \[{{V}_{1}}=|[\overset{\to }{\mathop{\alpha }}\,\overset{\to }{\mathop{\beta }}\,\overset{\to }{\mathop{\gamma }}\,]|\] \[\Rightarrow \]               \[{{V}_{1}}=\left| \begin{matrix}    \mathbf{\vec{a}}\cdot \mathbf{\vec{a}} & \mathbf{\vec{a}}\cdot \mathbf{\vec{b}} & \mathbf{\vec{a}}\cdot \mathbf{\vec{c}}  \\    \mathbf{\vec{a}}\cdot \mathbf{\vec{b}} & \mathbf{\vec{b}}\cdot \mathbf{\vec{b}} & \mathbf{\vec{b}}\cdot \mathbf{\vec{c}}  \\    \mathbf{\vec{a}}\cdot \mathbf{\vec{c}} & \mathbf{\vec{b}}\cdot \mathbf{\vec{c}} & \mathbf{\vec{c}}\cdot \mathbf{\vec{c}}  \\ \end{matrix} \right|[\mathbf{\vec{a}\vec{b}\vec{c}}]\] \[\Rightarrow \]               \[{{V}_{1}}={{[\mathbf{\vec{a}\vec{b}\vec{c}}]}^{2}}[\mathbf{\vec{a}\vec{b}\vec{c}}]\] \[\Rightarrow \]               \[{{V}_{1}}={{[\mathbf{\vec{a}\vec{b}\vec{c}}]}^{3}}\] \[\Rightarrow \,\,\,\,\,\,\,\,{{V}_{1}}={{V}^{3}}\]


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