RAJASTHAN ­ PET Rajasthan PET Solved Paper-2007

  • question_answer
    Let a, b and c are different non-negative numbers. If vectors\[a\hat{i}+a\hat{j}+c\hat{k},\text{ }\hat{i}+\hat{k}\] and \[c\hat{i}+c\hat{j}+b\hat{k}\]lies in the same plane, then the value of c is

    A)  geometric mean of a and b

    B)  arithmetic mean of a and b

    C)  zero

    D)  harmonic mean of a and b

    Correct Answer: A

    Solution :

     Given, points lie in a plane, if \[\left| \begin{matrix}    a & a & c  \\    1 & 0 & 1  \\    c & c & b  \\ \end{matrix} \right|=0\] Applying \[{{C}_{1}}\to {{C}_{1}}-{{C}_{2}}\] \[\Rightarrow \] \[\left| \begin{matrix}    0 & a & c  \\    1 & 0 & 1  \\    0 & c & b  \\ \end{matrix} \right|=0\] \[\Rightarrow \] \[-1[ab-{{c}^{2}}]=0\] \[\Rightarrow \] \[{{c}^{2}}=ab\] Hence, c is the geometric mean of o and b.


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