J & K CET Engineering J and K - CET Engineering Solved Paper-2006

  • question_answer
    If \[\int{\frac{dx}{x\,\log \,x}}=f(x)+\] constant,  then \[f(x)\] is equal to

    A)  \[1/\,\log \,x\]        

    B)  \[\log \,x\]

    C)  \[\log \,\,\log \,x\]

    D)  \[x/\,\log \,x\]

    Correct Answer: C

    Solution :

    We have \[\int{dx/(x\,\log x)=f(x)+\text{constant}}\] ??(i) Let \[\log \,x=t\] \[\Rightarrow \] \[\frac{1}{x}\,\,dx=dt\] \[\therefore \] \[\int{\frac{dx}{x\,\,\log \,x}}=\int{\frac{dt}{t}}=\log (t)+constant\] \[=\log \,(\log x)+\text{constant}\] On comparing with Eq. (i), we get \[f(x)=\log \,(\log x)\]


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