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

  • question_answer
    Given \[f(x)=b({{[x]}^{2}}+[x])+1\] for \[x\ge -1\]\[=\sin (\pi (x+a))\] for \[x<-1\]where [x] denotes the integral part of x, then for what values of a, b, the function is continuous at x = -1?

    A) \[a=2n+(3/2);b\in R;n\in I\]

    B) \[a=4n+2;b\in R;n\in I\]

    C) \[a=4n+(3/2);b\in {{R}^{+1}};n\in I\]

    D) \[a=4n+1;b\in {{R}^{+}};n\in I\]

    Correct Answer: A

    Solution :

    [a] \[f(-1)=b(1-1)+1=1;\underset{h\to 0}{\mathop{\lim }}\,f(-1+h)=1\] \[\underset{h\to 0}{\mathop{\lim }}\,f(-1-h)=\underset{h\to 0}{\mathop{\lim }}\,\sin (\pi (-1-h)+\pi a)\]                         \[=\sin (-\pi +\pi a)=-sin\pi a\] For continuous \[\sin \pi a=-1=\sin \left( 2n\pi +\frac{3\pi }{2} \right)\] \[\Rightarrow \pi a=2n\pi +\frac{3\pi }{2}\Rightarrow a=2n+\frac{3}{2}\] Hence, \[a=2n+\frac{3}{2},n\in I\] and \[b\in R\].


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