Manipal Engineering Manipal Engineering Solved Paper-2011

  • question_answer
    If\[{{\log }_{3}}2,\,\,{{\log }_{3}}({{2}^{x}}-5)\]and\[{{\log }_{3}}\left( {{2}^{x}}-\frac{7}{2} \right)\]are in AP, then\[x\]is equal to

    A) \[1,\,\,\frac{1}{2}\]                                        

    B) \[1,\,\,\frac{1}{3}\]

    C) \[1,\,\,\frac{3}{2}\]                        

    D)         None of these

    Correct Answer: D

    Solution :

    Since, it is an\[AP\]. \[\therefore \]  \[{{\log }_{3}}({{2}^{x}}-5)=\frac{{{\log }_{3}}2+{{\log }_{3}}\left( {{2}^{x}}-\frac{7}{2} \right)}{2}\] \[\Rightarrow \]               \[2{{\log }_{3}}({{2}^{x}}-5)={{\log }_{3}}\left( 2\cdot \left( {{2}^{x}}-\frac{7}{2} \right) \right)\] \[\Rightarrow \]               \[2{{\log }_{3}}{{({{2}^{x}}-5)}^{2}}={{2}^{x+1}}-7\] \[\Rightarrow \]               \[{{2}^{2x}}-12\cdot {{2}^{x}}-32=0\] \[\Rightarrow \]               \[x=2,\,\,3\] But\[x=2,\,\,{{\log }_{3}}({{2}^{x}}-5)\]is not defined.


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