CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2007

  • question_answer
    If a, b, c are distinct positive numbers each being different from 1 such that \[({{\log }_{b}}a.{{\log }_{c}}a-{{\log }_{a}}a)\]                                 \[+({{\log }_{a}}b.{{\log }_{c}}b-{{\log }_{b}}b)\] \[+({{\log }_{a}}c.{{\log }_{b}}c-{{\log }_{c}}c)=0,\]then \[abc\] is

    A)  \[0\]

    B)                                         \[e\]

    C)  \[1\]                    

    D)         \[2\]

    E)  \[3\]

    Correct Answer: C

    Solution :

    \[({{\log }_{b}}a{{\log }_{c}}a-{{\log }_{a}}a)+({{\log }_{a}}b{{\log }_{c}}b\]\[-{{\log }_{b}}b)+{{\log }_{a}}c.{{\log }_{b}}c-{{\log }_{c}}c)=0\] \[\Rightarrow \] \[\left( \frac{\log a}{\log b}.\frac{\log a}{\log c}-\frac{\log a}{\log a} \right)\]\[+\left( \frac{\log b}{\log a}.\frac{\log b}{\log c}-\frac{\log b}{\log b} \right)\] \[+\left( \frac{\log c}{\log a}.\frac{\log c}{\log b}-\frac{\log c}{\log c} \right)=0\] \[\Rightarrow \]\[\log a\left( \frac{{{(\log a)}^{2}}-\log b\log c}{\log a\log b\log c} \right)\]                                 \[+\log b\left( \frac{{{(\log b)}^{2}}-\log a\log c}{\log a\log b\log c} \right)\]                 \[+\log c\left( \frac{{{(\log c)}^{2}}-\log a\log b}{\log a\log b\log c} \right)=0\] \[\Rightarrow \]               \[{{(\log a)}^{3}}+{{(\log b)}^{3}}+{{(\log c)}^{3}}\]                                                 \[-3\log a\log b\log c=0\] \[\Rightarrow \]               \[(\log a+\log b+\log c)=0\]         \[\Rightarrow \]               \[abc=1\]


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