JEE Main & Advanced Mathematics Functions Question Bank Continuity

  • question_answer
    If\[f(x)=\left\{ \begin{align}   & \,\,\,\,\,\,\,\,\,\,\,\,\,\,1,\,\text{when}\,\,0<x\le \frac{3\pi }{4} \\  & 2\sin \frac{2}{9}x,\text{when}\,\frac{3\pi }{4}<x<\pi  \\ \end{align} \right.\], then                    [IIT 1991]

    A)            \[f(x)\]is continuous at \[x=0\]

    B)            \[f(x)\]is continuous at \[x=\pi \]

    C)            \[f(x)\]is continuous at \[x=\frac{3\pi }{4}\]

    D)            \[f(x)\]is discontinuous at \[x=\frac{3\pi }{4}\]

    Correct Answer: C

    Solution :

               Here \[f\,\left( \frac{3\pi }{4} \right)=1\] and \[\because f\]                    \[\underset{x\to 3\pi /4+}{\mathop{\lim }}\,f(x)=\underset{h\to 0}{\mathop{\lim }}\,\,\,2\sin \frac{2}{9}\,\left( \frac{3\pi }{4}+h \right)=2\,\sin \frac{\pi }{6}=1\].            Hence \[f(x)\] is continuous at \[\frac{\sin \,\,2x}{\sin \,\left( \frac{x}{2} \right)}\].


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