JEE Main & Advanced Mathematics Functions Question Bank Limits

  • question_answer
    \[\underset{x\to \pi /2}{\mathop{\lim }}\,\frac{{{a}^{\cot x}}-{{a}^{\cos x}}}{\cot x-\cos x}=\]                        [Kerala (Engg.) 2001; J & K 2005]

    A)                 \[\log a\]

    B)                 \[\log 2\]

    C)                 a

    D)                 log x

    Correct Answer: A

    Solution :

                       \[\underset{x\to \pi /2}{\mathop{\text{lim}}}\,\,\left( \frac{{{a}^{\cot x}}-{{a}^{\cos x}}}{\cot x-\cos x} \right)\]\[=\underset{x\to \pi /2}{\mathop{\text{lim}}}\,{{a}^{\cos x}}\left( \frac{{{a}^{\cot x-\cos x}}-1}{\cot x-\cos x} \right)\]                                 \[={{a}^{\cos (\pi /2)}}\underset{x\to \pi /2}{\mathop{\text{lim}}}\,\left( \frac{{{a}^{\cot x-\cos x}}-1}{\cot x-\cos x} \right)\]\[=1.\log a=\log a\].


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