JEE Main & Advanced Mathematics Indefinite Integrals Question Bank Mock Test - Integrals

  • question_answer
    If \[f(x)=\frac{{{e}^{x}}}{1+{{e}^{x}}},\,\,\,{{I}_{1}}=\int\limits_{f(-a)}^{f(a)}{xg\,(x(1-x))dx,}\] and \[{{I}_{2}}=\int\limits_{f(-a)}^{f(a)}{g(x(1-x))dx,}\] then the value of \[\frac{{{I}_{2}}}{{{I}_{1}}}\]is

    A) \[-\,1\]              

    B) \[-\,2\]

    C) 2                     

    D) 1

    Correct Answer: C

    Solution :

    [c] \[f(x)=\frac{{{e}^{x}}}{1+{{e}^{x}}}\] \[\therefore f(a)=\frac{{{e}^{q}}}{1+{{e}^{a}}}\] And \[f(-a)=\frac{{{e}^{-a}}}{1+{{e}^{-a}}}=\frac{{{e}^{-a}}}{1+\frac{1}{{{e}^{a}}}}=\frac{1}{1+{{e}^{a}}}\] \[\therefore f(a)+f(-a)=\frac{{{e}^{a}}+1}{1+{{e}^{q}}}=1\] Let \[f(-a)=\alpha \] or \[f(a)=1-\alpha \] Now, \[{{I}_{1}}=\int\limits_{\alpha }^{1-\alpha }{xg(x(1-x))dx}\] \[=\int\limits_{\alpha }^{1-\alpha }{(1-x)g((1-x)(1-(1-x))dx}\] \[=\int\limits_{\alpha }^{1-\alpha }{(1-x)g(x(1-x)dx}\] \[\therefore 2{{I}_{1}}=\int\limits_{\alpha }^{1-\alpha }{g(x(1-x))dx}={{I}_{2}}\] Or \[\frac{{{I}_{2}}}{{{I}_{1}}}=2\]


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