JEE Main & Advanced Mathematics Functions Question Bank Functions

  • question_answer
    If the function \[f:[1,\ \infty )\to [1,\ \infty )\] is defined by \[f(x)={{2}^{x(x-1)}},\] then \[{{f}^{-1}}\](x) is [IIT 1999]

    A)                    \[{{\left( \frac{1}{2} \right)}^{x(x-1)}}\]

    B)            \[\frac{1}{2}(1+\sqrt{1+4{{\log }_{2}}x})\]

    C)                    \[\frac{1}{2}(1-\sqrt{1+4{{\log }_{2}}x})\]

    D)            Not defined

    Correct Answer: B

    Solution :

               Given \[f(x)={{2}^{x(x-1)}}\,\Rightarrow \,\,x\,(x-1)={{\log }_{2}}f(x)\]            \[\Rightarrow \,\,{{x}^{2}}-x-{{\log }_{2}}f(x)=0\,\,\Rightarrow \,\,x=\frac{1\pm \sqrt{1+4{{\log }_{2}}f(x)}}{2}\]            Only \[x=\frac{1+\sqrt{1+4{{\log }_{2}}f(x)}}{2}\] lies in the domain                    \[\therefore \,\,{{f}^{-1}}(x)=\frac{1}{2}\,[1+\sqrt{1+4\,{{\log }_{2}}x}]\].


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