JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Geometry of complex numbers

  • question_answer
    The complex numbers \[z=x+iy\] which satisfy the equation \[\left| \frac{z-5i}{z+5i} \right|=1\] lie on [IIT 1982]

    A) Real axis

    B) The line \[y=5\]

    C) A circle passing through the origin

    D) None of these

    Correct Answer: A

    Solution :

    \[\left| \frac{z-5i}{z+5i} \right|=1\]Þ\[\left| \frac{x+i(y-5)}{x+i(y+5)} \right|=1\] Þ \[|x+i(y-5)|\,=\,|x+i(y+5)|\], \[\left( \because \left| \frac{{{z}_{1}}}{{{z}_{2}}} \right|=\frac{|{{z}_{1}}|}{|{{z}_{2}}|} \right)\] Þ  \[{{x}^{2}}+25-10y+{{y}^{2}}={{y}^{2}}+{{x}^{2}}+25+10y\] Þ \[20y=0\]  Þ   \[y=0\].


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