RAJASTHAN ­ PET Rajasthan PET Solved Paper-2005

  • question_answer
    If  function \[f(x)=\left\{ \begin{matrix}    {{x}^{2}}/a & ,0\le x<1  \\    a & ,1\le x<\sqrt{2}  \\    \frac{(2{{b}^{2}}-4b)}{{{x}^{2}}} & \sqrt{2}\le x<\infty   \\ \end{matrix} \right.\] is continuous for\[0<x<\infty ,\]then the values of\[a\] and\[b\]are

    A)  \[a=1,b=-1\]     

    B)  \[a=-1,b=1\]

    C)  \[a=-1,b\text{ }=1+\sqrt{2}\]

    D)  \[a=-1,b=\sqrt{2}-1\]

    Correct Answer: B

    Solution :

     Given,\[f(x)=\left\{ \begin{matrix}    {{x}^{2}}/a & , & 0\le x<1  \\    a & , & 1\le x<\sqrt{2}  \\    (2{{b}^{2}}-4b)/{{x}^{2}} & , & \sqrt{2}\le x<\infty   \\ \end{matrix} \right.\] At\[x=1,f(x)\]is continuous. \[\therefore \] \[f(1-0)=f(1)=f(1+0)\] \[\Rightarrow \] \[\frac{1}{a}=a\] \[\Rightarrow \] \[{{a}^{2}}=1\] \[\Rightarrow \] \[a=\pm 1\] At\[x=\sqrt{2},f(x)\]is continuous. \[\therefore \] \[f(\sqrt{2}-0)=f(\sqrt{2})=f(\sqrt{2}+0)\] \[\Rightarrow \] \[a=\frac{2{{b}^{2}}-4b}{2}\] \[\Rightarrow \] \[a={{b}^{2}}-2b\] When\[a=-1,\]then \[{{b}^{2}}-2b+1=0\] \[\Rightarrow \] \[{{(b-1)}^{2}}=0\] \[\Rightarrow \] \[b=1\] \[\therefore \] \[a=-1,b=1\] When\[a=1,\]then \[{{b}^{2}}-2b-1=0\] \[\Rightarrow \] \[b=1\pm \sqrt{2}\]


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