JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Self Evaluation Test - Complex Numbers and Quadratic Equations

  • question_answer
    If \[|z|=\max \,\{|z-1|,\,|z+1|\}\] then

    A) \[|z+\bar{z}|=\frac{1}{2}\]

    B) \[z+\bar{z}=1\]

    C) \[|z+\bar{z}|=1\]     

    D) None of these

    Correct Answer: D

    Solution :

    If \[\left| z-1 \right|>\left| z+1 \right|,\]then max \[\{|z-1|,|z+1|\}=|z-1|\] \[\Rightarrow \] If \[|z{{|}^{2}}+1-z-\bar{z}>|z{{|}^{2}}+1+z+\bar{z}\] then \[|z|=|z-1|\] \[\Rightarrow \] If \[z+\bar{z}<0\] then \[|z{{|}^{2}}=|z{{|}^{2}}+1-z-\bar{z}\] \[\Rightarrow \] If \[z+\bar{z}<0\] then \[z+\bar{z}=1,\] which is not possible. Again If \[|z+1|>|z-1|\] then max \[\{|z-1|,|z+1|\}=|z+1|\] \[\Rightarrow \]  If \[|z{{|}^{2}}+1+z+\bar{z}>\,|z{{|}^{2}}+1-z-\bar{z}\] then \[|z|\,=\,|z+1|\] \[\Rightarrow \]  If \[z+\bar{z}>0\] then \[|z{{|}^{2}}=|z{{|}^{2}}+1+z+\bar{z}\] \[\Rightarrow \] If \[z+\bar{z}>0\]then \[z+\bar{z}=-1\] Not possible again. Therefore the given result cannot hold.


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