J & K CET Engineering J and K - CET Engineering Solved Paper-2004

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

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

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

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

    D)  \[\infty \]

    Correct Answer: B

    Solution :

    Given that,   \[f:[1,-\infty )\xrightarrow{{}}[1,\,\infty )\] and  \[f(x)={{2}^{x(x-1)}}=y\] \[\Rightarrow \] \[{{\log }_{2}}y={{x}^{2}}-x\] \[\Rightarrow \] \[{{x}^{2}}-x-{{\log }_{2}}y=0\] \[\Rightarrow \] \[x=\frac{1\pm \sqrt{1+4\,{{\log }_{2}}y}}{2}\] \[\Rightarrow \] \[x=\frac{1+\sqrt{1+4\,{{\log }_{2}}y}}{2}\] (Neglect \[-ve\] sign due to the domain)


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