JEE Main & Advanced Sample Paper JEE Main Sample Paper-15

  • question_answer
    If \[[\overset{\to }{\mathop{p}}\,+2\overset{\to }{\mathop{q}}\,+3\overset{\to }{\mathop{r}}\,\overset{\to }{\mathop{q}}\,+2\overset{\to }{\mathop{r}}\,+3\overset{\to }{\mathop{p}}\,\,\overset{\to }{\mathop{r}}\,+2\overset{\to }{\mathop{p}}\,+3\overset{\to }{\mathop{q}}\,]=54,\]where \[\overset{\to }{\mathop{p}}\,,\overset{\to }{\mathop{q}}\,\] and \[\overset{\to }{\mathop{r}}\,\] are three vectors then the value of\[\left| \begin{matrix}    \overset{\to }{\mathop{p}}\,.\overset{\to }{\mathop{p}}\, & \overset{\to }{\mathop{p}}\,.\overset{\to }{\mathop{q}}\, & \overset{\to }{\mathop{p}}\,.\overset{\to }{\mathop{r}}\,  \\    \overset{\to }{\mathop{p}}\,.\overset{\to }{\mathop{q}}\, & \overset{\to }{\mathop{q}}\,.\overset{\to }{\mathop{q}}\, & \overset{\to }{\mathop{q}}\,.\overset{\to }{\mathop{r}}\,  \\    \overset{\to }{\mathop{p}}\,.\overset{\to }{\mathop{r}}\, & \overset{\to }{\mathop{r}}\,.\overset{\to }{\mathop{q}}\, & \overset{\to }{\mathop{r}}\,.\overset{\to }{\mathop{r}}\,  \\ \end{matrix} \right|\] is

    A)  4                                            

    B)  9

    C)  16                                         

    D)  36

    Correct Answer: B

    Solution :

    \[(\overrightarrow{p}+2\overrightarrow{q}+3\overrightarrow{r})\cdot \left( (\overrightarrow{q}+2\overrightarrow{r}+3\overrightarrow{p})\times (\overrightarrow{r}+2\overrightarrow{p}+3\overrightarrow{q}) \right)=54\]On solving \[18\left[ \overrightarrow{p}\,\,\,\overrightarrow{q}\,\,\,\overrightarrow{r} \right]=54\]\[\left[ \overrightarrow{p}\,\,\,\overrightarrow{q}\,\,\,\overrightarrow{r} \right]=3\] Hence,\[{{\left[ \overrightarrow{p}\,\,\,\overrightarrow{q}\,\,\,\overrightarrow{r} \right]}^{2}}=9\]Ans.


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