JEE Main & Advanced JEE Main Paper (Held On 25 April 2013)

  • question_answer
                    Let z satisfy \[\left| z \right|=1\] and \[\left| z \right|=1-\overset{-}{\mathop{z}}\,\]                     Statement I : z is a real number                 Statement II : Principal argument of\[z\] is \[\frac{\pi }{3}\]     JEE Main Online Paper ( Held On 25  April 2013 )

    A)                 Statement I true: Statement II is true; Statement II is a correct explanation for statement I                

    B)                 Statement I is false; Statement II is true.

    C)                 Statement I is true; statement II is false.                

    D)                 Statement I is true; Statement II is true; Statement II is not a correct explanation for Statement I.

    Correct Answer: B

    Solution :

                    Let\[z=x+iy,\overline{z}=x-iy\] Now, \[z=1-\overline{z}\] \[\Rightarrow \]\[x+iy=1-(x-iy)\] \[\Rightarrow \]\[2x=1\Rightarrow x=\frac{1}{2}\] Now,\[|z|=1\Rightarrow {{x}^{2}}+{{y}^{2}}=1\Rightarrow {{y}^{2}}=1-{{x}^{2}}\] \[\Rightarrow \]\[y=\pm \frac{\sqrt{3}}{2}\] Now, \[\tan \theta =\frac{y}{x}\](\[\theta \] is the argument) \[=\frac{\sqrt{3}}{2}\div \frac{1}{2}\](+ve since only principal argument) \[=\sqrt{3}\]\[\Rightarrow \]\[\theta ={{\tan }^{-1}}\sqrt{3}=\frac{\pi }{3}\] Hence, z is not a real number So, statement-1 is false and 2 is true.                


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