JEE Main & Advanced Sample Paper JEE Main Sample Paper-16

  • question_answer
    If \[\int_{{}}^{{}}{f(x)dx=g(x)}\], then \[\int_{{}}^{{}}{{{f}^{-1}}}(x)dx\] is

    A) \[x\,{{f}^{-1}}(x)+C\]                     

    B) \[f\left( {{g}^{-1}}(x) \right)+C\]

    C) \[x{{f}^{-1}}(x)-g\left( {{f}^{-1}}(x) \right)+C\]

    D)  \[{{g}^{-1}}(x)+C\]  [Note: Where 'C' is constant of integration.]

    Correct Answer: C

    Solution :

    \[I=\int_{{}}^{{}}{{{f}^{-1}}}(x)dx\,put\,{{f}^{-1}}(x)=t\] \[\Rightarrow \]\[x=f(t)\]or\[dx=f'(t)dt\] \[\Rightarrow \]\[I=\int_{{}}^{{}}{t\cdot f'(t)dt=t\,f(t)-\int_{{}}^{{}}{1\cdot f(t)dt}}\] \[=t\,f\,(t)-g(t)+C\] \[={{f}^{-1}}(x)x-g\left( {{f}^{-1}}(x) \right)+C\]


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