JEE Main & Advanced JEE Main Solved Paper-2016

  • question_answer
    A value of\[\theta \] for which\[\frac{2+3i\sin \theta }{1-2i\sin \theta }\]is purely imaginary, is : [JEE Main Solved Paper-2016 ]

    A) \[{{\sin }^{-1}}\left( \frac{1}{\sqrt{3}} \right)\]                  

    B) \[\frac{\pi }{3}\]

    C) \[\frac{\pi }{6}\]                                              

    D) \[{{\sin }^{-1}}\left( \frac{\sqrt{3}}{4} \right)\]

    Correct Answer: A

    Solution :

                    \[|Z=\frac{2+3i\sin \theta }{1-2\sin \theta }\] \[\Rightarrow \]\[|Z=\frac{(2+3i\sin \theta )(1+2i\sin \theta )}{1+4{{\sin }^{2}}\theta }\] \[=\frac{\left( 2-6i{{\sin }^{2}}\theta  \right)+7i\sin \theta )}{1+4{{\sin }^{2}}\theta }\] for purely imaginary Z, Re(Z) = 0 \[\Rightarrow \] \[2-6{{\sin }^{2}}\theta =0\Rightarrow \sin \theta =\pm \frac{1}{\sqrt{3}}\] \[\Rightarrow \]\[\theta =\pm {{\sin }^{-1}}\left( \frac{1}{\sqrt{3}} \right)\]


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