CEE Kerala Engineering CEE Kerala Engineering Solved Paper-2006

  • question_answer
    If\[\int{x}\,f(x)dx=\frac{f(x)}{2},\]then\[f(x)\]is equal to:

    A)  \[{{e}^{x}}\]                                     

    B)  \[{{e}^{-x}}\]

    C)  \[\log x\]           

    D)         \[\frac{{{e}^{{{x}^{2}}}}}{2}\]

    E)  \[{{e}^{{{x}^{2}}}}\]

    Correct Answer: D

    Solution :

    Let us assume\[f(x)=\frac{{{e}^{{{x}^{2}}}}}{2}\] \[\therefore \]\[\int{x.}\frac{{{e}^{{{x}^{2}}}}}{2}dx=\int{2x\frac{{{e}^{{{x}^{2}}}}}{4}}dx\] Let \[{{x}^{2}}=t\] \[\Rightarrow \]               \[2x\,dx=dt\] \[\therefore \]  \[\int{\frac{{{e}^{t}}}{4}}dt=\frac{{{e}^{t}}}{4}+c=\frac{1}{2}\left( \frac{{{e}^{{{x}^{2}}}}}{2} \right)+c\] \[\therefore \]\[\int{x}f(x)dx=\frac{1}{2}f(x)+c\]when \[f(x)=\frac{{{e}^{{{x}^{2}}}}}{2}.\]


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