JEE Main & Advanced Mathematics Functions Question Bank Critical Thinking

  • question_answer
    If \[f(x)=\left\{ \begin{align}   & {{x}^{2}}-3,\ 2<x<3 \\  & 2x+5,\ 3<x<4 \\ \end{align} \right.\], the equation whose roots are \[\underset{x\to {{3}^{-}}}{\mathop{\lim }}\,f(x)\]and\[\underset{x\to {{3}^{+}}}{\mathop{\lim }}\,f(x)\]is [Orissa JEE 2004]

    A)            \[{{x}^{2}}-7x+3=0\]

    B)            \[{{x}^{2}}-20x+66=0\]

    C)            \[{{x}^{2}}-17x+66=0\]

    D)            \[{{x}^{2}}-18x+60=0\]

    Correct Answer: C

    Solution :

               \[f(x)=\left\{ \begin{align}   & {{x}^{2}}-3,\,\,2<x<3 \\  & 2x+5,\,3<x<4 \\ \end{align} \right.\]                    \\[\underset{x\to {{3}^{-}}}{\mathop{\lim }}\,f(x)=\underset{x\to {{3}^{-}}}{\mathop{\lim }}\,({{x}^{2}}-3)=6\]                    and \[\underset{x\to {{3}^{+}}}{\mathop{\lim }}\,f(x)=\underset{x\to {{3}^{+}}}{\mathop{\lim }}\,(2x+5)=11\]                    Hence, the required equation will be                    \[{{x}^{2}}-\](sum of roots) x+ (Product of roots) = 0                    i.e., \[{{x}^{2}}-17x+66=0\].


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