JCECE Engineering JCECE Engineering Solved Paper-2007

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

    A)  the axis of\[x\]

    B)  the straight line\[y=5\]

    C)  the circle passing through the origin

    D)  none of the above

    Correct Answer: A

    Solution :

    Given,\[z=x+iy\]and \[\left| \frac{z-5i}{z+5i} \right|=1\]         \[\because \]     \[\left| \frac{{{z}_{1}}}{{{z}_{2}}} \right|=\frac{|{{z}_{1}}|}{|{{z}_{2}}|}\] \[\Rightarrow \]               \[\frac{|x+iy-5i|}{|x+iy-5i|}=1\] \[\Rightarrow \]               \[|x+iy-5i|=|x+iy+5i|\] \[\Rightarrow \]               \[{{x}^{2}}+{{(y-5)}^{2}}={{x}^{2}}+{{(5+y)}^{2}}\] \[\Rightarrow \]               \[-10y=10y\Rightarrow y=0\] Which is the equation of axis of\[x\].


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