JEE Main & Advanced Mathematics Relations Question Bank Mock Test - Relations and Functions

  • question_answer
    The domain of the functions \[f(x)=\frac{1}{\sqrt{\left\{ \operatorname{sinx} \right\}+\left\{ \sin (\pi +x) \right\}}}\] where \[\left\{ \cdot  \right\}\] denotes the fractional part, is

    A) \[[0,\pi ]\]          

    B) \[(2n+1)\pi /2,n\in Z\]

    C) \[(0,\pi )\]          

    D) none of these

    Correct Answer: D

    Solution :

    [d] \[f(x)=\frac{1}{\sqrt{\{sinx\}+\{sin(\pi +x)\}}}\] \[=\frac{1}{\sqrt{\{sinx\}+\{-sinx\}}}\] Now, \[\{\sin \,x\}+\{-\sin x\}=\left\{ \begin{matrix}    0,\,\,\,\,\,\,\,\sin \,x\,\,\text{is}\,\text{an}\,\text{integer}  \\    1,\,\,\sin x\,\text{is}\,\text{not}\,\text{an}\,\text{integer}  \\ \end{matrix} \right.\]For \[f(x)\] to get defined, \[\{sinx\}+\{-sinx\}\ne 0\] Or \[\sin x\ne \operatorname{int}eger\] Or \[\sin x\ne \pm 1,0\] Or \[x\ne \frac{n\pi }{2},n\in I\] Hence, the domain is\[R-\left\{ \frac{n\pi }{2}/n\in I \right\}\].


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