JEE Main & Advanced Mathematics Complex Numbers and Quadratic Equations Question Bank Conjugate, Modulus and Argument of complex number

  • question_answer
    If \[\frac{2{{z}_{1}}}{3{{z}_{2}}}\] is a purely imaginary number, then \[\left| \frac{{{z}_{1}}-{{z}_{2}}}{{{z}_{1}}+{{z}_{2}}} \right|\] =  [MP PET 1993]

    A) 3/2

    B) 1

    C) 2/3

    D) 4/9

    Correct Answer: A

    Solution :

    As given, let \[\frac{2{{z}_{1}}}{3{{z}_{2}}}=iy\]or \[\frac{{{z}_{1}}}{{{z}_{2}}}=\frac{3}{2}iy\], so that \[\left| \frac{{{z}_{1}}-{{z}_{2}}}{{{z}_{1}}+{{z}_{2}}} \right|=\left| \frac{\frac{{{z}_{1}}}{{{z}_{2}}}-1}{\frac{{{z}_{1}}}{{{z}_{2}}}+1} \right|=\left| \frac{\frac{3}{2}iy-1}{\frac{3}{2}iy+1} \right|=\left| \frac{1-\frac{3}{2}iy}{1+\frac{3}{2}iy} \right|=1\] \[\left\{ \because \,\,\,\,|z|\,=\,|\overline{z}| \right\}\]


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