JEE Main & Advanced Sample Paper JEE Main Sample Paper-7

  • question_answer
    If the line passing through (-2, 1, b) and (4, 1, 2) is perpendicular to the vector \[\hat{i}+3\hat{j}-2\hat{k}\] and is parallel to the plane containing the vectors \[\hat{i}+c\hat{k}\] and \[c\hat{j}+b\hat{k},\] then the ordered pair \[(b,\,c)\] is

    A)  \[(-1,\,-1/2)\]                  

    B)  \[(1,\,-6)\]

    C)  \[(-1,\,0)\]                        

    D)  \[(1,\,1/2)\]

    Correct Answer: C

    Solution :

    Vector along the line is \[\vec{V}=6\hat{i}+0.\hat{j}+(2-b)\hat{k}\] It is perpendicular to vector \[{{\vec{V}}_{1}}=\hat{i}+3\hat{j}-2\hat{k}\] \[\therefore \,\,\vec{V}\cdot {{\vec{V}}_{1}}=0\] \[\Rightarrow \] \[6-2(a-b)=0\] \[\Rightarrow \] \[b=-1\] Now the vector perpendicular to the vector \[\hat{i}+c\hat{k}\] and \[c\hat{j}+b\hat{k}\] is  \[\vec{n}=\left| \begin{matrix}    {\hat{i}} & {\hat{j}} & {\hat{k}}  \\    1 & 0 & c  \\    0 & c & b  \\ \end{matrix} \right|=-{{c}^{2}}\hat{i}-b\hat{j}+c\hat{k}\]. \[\therefore \,\,\vec{u}.\vec{n}=0\] \[\Rightarrow \,-6{{c}^{2}}+3c=0\] \[\Rightarrow \,c=0,\,\frac{1}{2}\]. \[\therefore \,\,(b,c)=(-1,\,0)\] or \[\left( -1,\,\frac{1}{2} \right)\]


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