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

  • question_answer
    If the derivative of the function\[f(x)=\left\{ \begin{matrix}    a{{x}^{2}}+b & x<-1  \\    b{{x}^{2}}+ax+4 & x\ge -1  \\ \end{matrix} \right.\]is everywhere continuous, then what are the values of a and b?

    A) a=2, b=3

    B) a=3, b=2

    C) a=-2, b=-3

    D) a=-3, b=-2

    Correct Answer: A

    Solution :

    [a] Derivative of \[f(x)=\left\{ \begin{matrix}    a{{x}^{2}}+b & x<-1  \\    b{{x}^{2}}+ax+a & x\ge -1  \\ \end{matrix} \right.\] is \[f'(x)=\left\{ \begin{matrix}    2ax & x<-1  \\    2bx+a, & x\ge -1  \\ \end{matrix} \right.\] If \[f'(x)\] is continuous everywhere then it is also continuous at \[x=-1\] \[{{\left. f'(x) \right|}_{x=-1}}=-2a=-2b+a\] or, \[3a=2b\]                                          ? (i) From the given choice \[a=2,b=3\] satisfied this equation.


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