JEE Main & Advanced Sample Paper JEE Main Sample Paper-40

  • question_answer
    If r(x) = f (x) and  f(0) =2, then \[\int_{{}}^{{}}{\frac{f(x)}{3+4f(x)}}dx=\]

    A) \[\log (3+8{{e}^{x}})+C\] 

    B) \[\frac{1}{4}\log (3+8{{e}^{x}})+C\]

    C) \[\frac{1}{2}\log (3+8{{e}^{x}})+C\]      

    D) none of these

    Correct Answer: B

    Solution :

    Since, f'(x)=f(x), therefore, \[f(x)=\alpha {{e}^{x}}\] Since, f(0) = 2, therefore, \[f(x)=2{{e}^{x}}\] \[\therefore \]\[I=2\int_{{}}^{{}}{\frac{{{e}^{x}}}{3+8{{e}^{x}}}}dx\]Put\[{{e}^{x}}=t,\therefore dt={{e}^{x}}dx\] \[\therefore \]\[I=2\int_{{}}^{{}}{\frac{dt}{3+8t}\Rightarrow I=\frac{1}{4}\log (3+8{{e}^{x}})+C}\]


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