JEE Main & Advanced Mathematics Limits, Continuity and Differentiability Question Bank Mock Test - Continuity and Differentiability

  • question_answer
    If \[f(x)=\left\{ \begin{matrix}    {{x}^{2}}-ax+3,\,\,\,x\,\,\text{is}\,\,\text{rational}  \\    2-x,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,x\,\,\text{is}\,\,\text{irrational}  \\ \end{matrix} \right.\]is continuous at exactly two points, then the possible values of a are

    A) \[(2,\infty )\]      

    B) \[(-\infty ,3)\]

    C) \[(-\infty ,-3)\cup (3,\infty )\]

    D) none of these

    Correct Answer: C

    Solution :

    [c] \[f(x)=\left\{ \begin{matrix}    {{x}^{2}}-ax+3,\,\,\,  \\    2-x,\,  \\ \end{matrix} \right.\begin{matrix}    x\,\,is\,\,rational  \\    x\,\,\text{is}\,\,\text{irrational}  \\ \end{matrix}\] It is continuous when \[{{x}^{2}}-ax+3=2-x\]or \[{{x}^{2}}-(a-1)x+1=0\] Which must have two distinct roots for \[{{(a-1)}^{2}}-4>0\] Or \[(a-1-2)(a-1+2)>0\] Or \[a\in (-\infty ,-1)\cup (3,\infty )\]


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