JEE Main & Advanced Mathematics Functions Question Bank Limits

  • question_answer
    The value of \[\underset{x\to 0}{\mathop{\lim }}\,{{\left( \frac{{{a}^{x}}+{{b}^{x}}+{{c}^{x}}}{3} \right)}^{2/x}}\]; \[(a,\ b,\ c>0)\] is

    A)                 \[{{(abc)}^{3}}\]

    B)                 \[abc\]

    C)                 \[{{(abc)}^{1/3}}\]

    D)                 None of these

    Correct Answer: D

    Solution :

                       Let \[y=\underset{x\to 0}{\mathop{\lim }}\,\,{{\left( \frac{{{a}^{x}}+{{b}^{x}}+{{c}^{x}}}{3} \right)}^{2/x}}\]            \[=\underset{h\to 0}{\mathop{\lim }}\,h\,\sin 1/h=0\times (-1\le \sin 1/h\le 1)=0\]                    \[=2\,\underset{x\to 0}{\mathop{\lim }}\,\,\frac{\log \,({{a}^{x}}+{{b}^{x}}+{{c}^{x}})-\log 3}{x}\]            Now applying L-Hospital?s rule, we have                 \[\log y=\log \,{{(abc)}^{2/3}}\,\Rightarrow \,\,y={{(abc)}^{2/3}}\]


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