KVPY Sample Paper KVPY Stream-SX Model Paper-27

  • question_answer
    Let \[\vec{a},\,\] \[\vec{b},\] \[\vec{c}\] are three non-coplanar vectors such that \[{{\vec{r}}_{1}}=\vec{a}-\vec{b}+\vec{c},\] \[{{\vec{r}}_{2}}=\vec{b}+\vec{c}-\vec{a},\] \[{{\vec{r}}_{3}}=\vec{c}+\vec{a}-\vec{b},\]\[\vec{r}=2\vec{a}-3\vec{b}+4\vec{c},\]  if \[\vec{r}={{\lambda }_{1}}{{\vec{r}}_{1}}+{{\lambda }_{2}}{{\vec{r}}_{2}}+{{\lambda }_{3}}{{\vec{r}}_{3}}\] then-

    A) \[{{\lambda }_{1}}=7\]

    B) \[{{\lambda }_{1}}+{{\lambda }_{3}}=3\]

    C) \[{{\lambda }_{1}}+{{\lambda }_{2}}+{{\lambda }_{3}}=3\]

    D) None of these

    Correct Answer: D

    Solution :

    \[2\vec{a}-3\vec{b}+4\vec{c}\]
    \[=({{\lambda }_{1}}-{{\lambda }_{2}}+{{\lambda }_{3}})\,\,\vec{a}+(-\,{{\lambda }_{1}}+{{\lambda }_{2}}-{{\lambda }_{3}})\,\,\vec{b}+({{\lambda }_{1}}+{{\lambda }_{2}}+{{\lambda }_{3}})\,\,\vec{c}\]Now \[{{\lambda }_{1}}-{{\lambda }_{2}}+{{\lambda }_{3}}=2,\]
    \[-\,{{\lambda }_{1}}+{{\lambda }_{2}}-\,{{\lambda }_{3}}=-\,3\]        \[\Rightarrow {{\lambda }_{1}}-{{\lambda }_{2}}+{{\lambda }_{3}}=3\]
    Clearly, these two condition can?t be true together
    So \[\lambda \in \phi \]


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